Fix cancellation in named-query and recipe searches

[Widthdom]

Summary

Named-query and recipe search discarded caller cancellation before reaching database queries. Propagate the token through a shared search database scope, interrupt running SQLite work, and check cancellation while materializing rows and aggregations. Issue-draft exports had the same database handoff gap and are covered too.

Cancellation remains an interruption across direct calls, CLI and serial/parallel batches, and the inherited reader restores its previous token after a cancelled nested query. Existing per-query failure handling must not turn cancellation into a degraded success or zero count.

Fixes #5427

Validation

• Debug cancellation suite: 7 passed on each of .NET 8 and .NET 9.
• Release solution build: no warnings or errors.
• make lint: passed, including cancellation test indentation.
• Release cancellation suite: 7 passed on each of .NET 8 and .NET 9.
• Release search/audit/batch and related regression suites: .NET 8 — 1,051 passed, 1 skipped; .NET 9 — 1,031 passed, 21 skipped.
• dotnet run --project tools/CodeIndex.Changelog -- check: 19 fragments validated.
• git diff --check: passed.
• Post-commit Debug rebuild and root/workspace index checks: passed.

Codex adversarial review of origin/main..HEAD: No blocking/actionable issues found. The review independently reran the 7 cancellation tests, 12 related regression tests, and the fragment validator successfully.

A second Codex review confirmed that the CI formatting correction changes only indentation, preserves behavior, and passes whitespace-format verification.

Documentation

• Updated English and Japanese sections in USER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5427.fixed.md; CHANGELOG.md is unchanged.
• Agent entry points and AGENT_GUIDE.md are unchanged.

No additional issues or follow-up findings are included.