docs: refresh compatibility milestone and implemented feature matrix

[fglock]

Summary

• Add the October 3 compatibility milestone to the README, roadmap, testing guide, and unreleased changelog: all 575 selected imported core test files completed without unexpected failures in the recorded macOS default-mode run.
• Disclose skipped assertions and skip-all files, TODO checks, excluded files, disabled large-memory cases, the planned/reported TAP count discrepancy, and the limits of interpreter and platform verification. Avoid an unqualified claim that the entire standard Perl suite passes.
• Refresh the feature matrix against current implementation and existing regression coverage: completed regex capability families, format expression execution and lexical captures, warnings/strict/UTF-8/bytes scoping, debugger modules and switches, typeglob localization, constructor validation, bitwise and file-test overloads, Storable canonical output, and native async/await.
• Explain the runtime configuration advertised by Config.pm: 64-bit integers, double precision and its integer conversion limit, threads/multiplicity, host identity, and no native or pseudo-fork. Retain documented native-access, XS, IPC, and backend boundaries.

Validation

• Documentation changes only (five Markdown files).
• nice -n 19 make check-links: passed, 0 errors.
• git diff --check: passed.
• No rebuild, unit suite, or UAT rerun; existing PR Fix failures in the top 200 core tests #1606 run evidence is documented with its original source commit and CI URL.

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