feat: stage cold appends before the claim, and stream a tiered INSERT's cold rows

[vyruss]

• With coldfront.iceberg_async_parquet and coldfront.iceberg_bakery_patch on, an INSERT or COPY into the cold tier, and the archiver's exports, upload their Parquet inside the statement and take the table's claim at commit, for the catalog commit alone: an advisory lock on a single node, the bakery claim on a mesh. An open transaction no longer blocks other writers, and statement_timeout still bounds the wait.
• A DELETE, UPDATE, MERGE, the cross-tier move and vector_train keep taking the claim first, since their position deletes name data files a concurrent compaction could rewrite.
• A tiered INSERT or COPY streams its cold rows: their projection runs in PostgreSQL over coldfront.local_pg_dsn, at the statement's own snapshot, and DuckDB writes them in one pass. Both tiers see one state of the source, and a REPEATABLE READ transaction reads its snapshot on both.
• A source the stream cannot read the same way twice goes through coldfront._cold_sink, which now gathers each batch with string_agg, so its cost grows linearly with the row count.
• Every value a cold write renders as text is in one canonical form, so the caller's DateStyle, IntervalStyle and extra_float_digits do not change what the cold tier stores.
• An append to a clustered vector table in the async ordering is checked at commit against the live centroid generation: a retrain that committed while its rows were being assigned fails the transaction with serialization_failure, for the client to retry, so no row lands with a cluster of a replaced generation.

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📝 Walkthrough

Walkthrough

The change adds tiered INSERT streaming with an in-session sink fallback, canonical rendering for cold values, and deferred claim acquisition for eligible appends. It updates concurrency tests, regression coverage, formal models, and documentation for these paths.

Changes

Cold write paths

Layer / File(s)	Summary
Tiered INSERT streaming and fallback extension/coldfront/src/coldfront.c, extension/coldfront/coldfront--1.0.sql, extension/coldfront/test/sql/tiered_insert_*, extension/coldfront/Makefile, docs/architecture*.md, docs/usage.md, docs/changelog.md	Eligible tiered INSERTs stream cold rows through a second PostgreSQL session using a shareable snapshot. Other sources use the in-session batched sink. Regression tests cover stream eligibility, snapshots, and fallback cases.
Canonical cold values and cross-tier moves extension/coldfront/coldfront--1.0.sql, extension/coldfront/src/coldfront.c, extension/coldfront/test/sql/cold_render_canonical.sql, docs/changelog.md, docs/usage.md	Cold-write values use canonical text and JSON rendering across session settings. Cross-tier moves route the cold operation through _exec_iceberg_with_claim and canonicalize captured rows.
Deferred claims for cold appends extension/coldfront/src/coldfront.c, extension/coldfront/coldfront--1.0.sql, extension/coldfront/test/sql/async_write_before_claim.sql, ci/journey.sh, docs/formal/*, docs/architecture*.md, docs/compaction.md, docs/installation.md, docker/entrypoint.sh, CLAUDE.md	On patched builds, eligible appends stage before queuing a claim for PRE_COMMIT. The callback acquires queued claims before the Iceberg commit; other writes use claim-first ordering. Tests and documentation describe the ordering and its concurrency behavior.
Clustered append generation checks extension/coldfront/coldfront--1.0.sql, extension/coldfront/test/sql/clustered_append_generation.sql, docs/architecture_vectors.md, docs/usage_vectors.md	Clustered appends pin the centroid generation used during staging. Commit verification raises serialization_failure if retraining changes that generation before commit.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

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Merge Risk: ⚪ Minimal · up to 14de5

The change defers claim acquisition for cold appends and streams tiered INSERT rows. The one open test-expectation concern has been fixed, and no remaining merge-blocking issue was found.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 32 functions across 2 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check		The description directly summarizes the cold-append claim timing, tiered INSERT streaming, fallback behavior, canonical rendering, and clustered-vector generation checks covered by the changeset.
Title check		The title clearly identifies the two primary changes: staging cold appends before acquiring the claim and streaming cold rows for tiered INSERT operations.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Full details: Docstring Coverage

Explanation

Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 32 functions across 2 files. (1 skipped: 1 unsupported.)

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🧹 Nitpick comments (1)

ci/journey.sh (1)

2424-2424: 🎯 Functional Correctness | 🔵 Trivial | ⚡ Quick win

Make TC-185 require an age-eligible orphan.

[0-9]+ accepts 0. The compactor prints deleted 0 orphan file(s)... when it deletes no files, so this assertion can pass without testing orphan cleanup. The setup creates fresh referenced writes and one fresh staged write; it does not create an age-eligible orphan under the default 72-hour filter.

Create an age-eligible orphan for TC-185, then require a nonzero deletion count. Alternatively, assert the expected preservation of the fresh staged file.

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Review comment at @ci/journey.sh at line 2424:
Update the TC-185 case in compactor_runs_beside_write to create an orphan
eligible under the default age filter and require a nonzero deletion count;
alternatively, assert that the fresh staged file is preserved.

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Inline comments:
Review comments at @docs/architecture.md:
- Line 364: Ensure clustered appends cannot commit a cluster assignment computed
against a stale generation: disable async ordering for clustered appends, or
acquire the claim before assignment and recompute the cluster under that claim
before finalizing the staged Parquet. Update the flow involving _vec_list_expr
and _exec_iceberg_with_claim; refreshing only the catalog generation is
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Nitpick comments:
Review comments at @ci/journey.sh:
- Line 2424: Update the TC-185 case in compactor_runs_beside_write to create an
orphan eligible under the default age filter and require a nonzero deletion
count; alternatively, assert that the fresh staged file is preserved.

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📒 Files selected for processing (24)

• CLAUDE.md
• ci/journey.sh
• docker/entrypoint.sh
• docs/architecture.md
• docs/architecture_decoupled.md
• docs/architecture_tiered.md
• docs/architecture_vectors.md
• docs/changelog.md
• docs/compaction.md
• docs/formal/Bakery_async_samenode.cfg
• docs/formal/Bakery_async_single.cfg
• docs/formal/README.md
• docs/installation.md
• docs/usage.md
• extension/coldfront/Makefile
• extension/coldfront/coldfront--1.0.sql
• extension/coldfront/src/coldfront.c
• extension/coldfront/test/sql/async_write_before_claim.sql
• extension/coldfront/test/sql/cold_render_canonical.sql
• extension/coldfront/test/sql/cold_write_batch_size_guc.sql
• extension/coldfront/test/sql/param_cold_via_plpgsql.sql
• extension/coldfront/test/sql/tiered_insert_isolation.sql
• extension/coldfront/test/sql/tiered_insert_single_pass.sql
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Inline comments:
Review comments at @docs/architecture.md:
- Line 364: Update the async-mode documentation to distinguish bakery conflicts
from the clustered-append generation check: state that bakery conflicts require
no application-level retry, and qualify the identical-behavior/performance-knob
claim to writes that do not use that check. Preserve the documented
serialization_failure and retry guidance for clustered appends.

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• extension/coldfront/test/expected/clustered_append_generation.out is excluded by !**/*.out
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• docs/architecture_vectors.md
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• docs/changelog.md
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• extension/coldfront/coldfront--1.0.sql

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