Avoid copying unchanged string pages during collection - #36

Reviewed e925f72 against its stacked base a4eb486 (#35). Found one Major issue: the mutation bit is cleared even when the native Vec runtime defers compaction. Once the pin is released, an otherwise unchanged page no longer gets retried, retaining overwritten storage until a later write. Please address this before merging.

Separating mutation tracking from byte-accounting dirtiness is appropriate, and the Ruby value regressions passed. The finding concerns reclamation after a deferred native compaction; details and a suggested regression are inline.

Validation:

• Fresh O2 head build with the CI-pinned Mere 991f299b compiler and mgz revision; compared with the existing O2 build of the exact #35 base.
• The new regression matched Ruby 4.0.9 on both revisions with indexing ON/OFF, normally and with MERE_RUBY_GC_STRESS=100 (eight runs).
• Five related corpus programs covering collections inside methods, suspended Fiber values, string-page reuse, collected handles, and exit roots matched Ruby on the head with indexing ON/OFF (ten runs).
• A controlled native contract test using the exact generated Vec runtime and a forced pin predicate confirmed that vec_compact can return without changing the arena. Applying the new flag-clearing logic retained 8,392,720 bytes after unpinning; an unconditional retry reduced the page to 4,096 bytes. This isolates the runtime contract rather than claiming a measured Ruby-level leak from the supplied corpus.
• Two default benchmark runs per revision matched Ruby's stdout. Timed collections were 0.044–0.054 s on #35 and 0.0322–0.0324 s on the head. Other validation ran concurrently; these are local workload measurements.
• CI for this exact head reports 510/510 corpus programs matching Ruby and a successful Linux build. Broader jobs remain pending. I did not repeat the entire corpus, Linux validation, or Unicode measurements locally.
• git diff --check passed.

Re-reviewed 8c85dd5 against the previously reviewed e925f72, retaining #35 as the stacked base. The previous Major finding is resolved. No new actionable findings.

The page's mutation flag is now cleared only when the production helper observes a strict capacity decrease. Native pin deferrals and equal-capacity copies conservatively remain pending, so a later collection can retry without another page write. This is compatible with the pinned compiler's unit-returning compaction API; equal-capacity copies may still be retried, as documented.

Validation:

• Fresh O2 build with the pinned Mere 991f299b compiler and mgz revision.
• unittest/string_page_compact_check.py passed using the actual production helper and generated runtime: both native early-return paths preserve pending work, unpinning reclaims the overwritten 8 MiB down to 4 KiB, and the live value remains intact.
• The string-page corpus regression matched Ruby 4.0.9 with indexing ON/OFF, normally and with MERE_RUBY_GC_STRESS=100.
• Related collected-handle and #35 exit-root regressions matched Ruby in both index modes.
• CI for this exact head reports 510/510 corpus programs matching Ruby and a successful Linux build. Broader jobs remain pending.
• git diff --check passed.

The deferred-compaction finding can be considered addressed. Full corpus, Linux execution, Unicode-suite timings, and benchmarks were not repeated locally.