fix(collections): visit wrapped collision entries once in retain

[bobzhang]

When a collision chain wraps around the end of the backing table, HashMap::retain and HashSet::retain can call the predicate more than once for an entry moved by deletion.

Start scanning at an empty slot so each wrapped chain is visited together. Add regressions that count visits and check that retained keys remain accessible; document the once-per-entry predicate contract.

Validation:

• moon check --deny-warn
• moon test hashmap hashset --target all
• moon info; generated interfaces unchanged

[coveralls]

Coverage Report for CI Build 6623

Coverage increased (+0.004%) to 89.318%

Details

• Coverage increased (+0.004%) from the base build.
• Patch coverage: 8 of 8 lines across 2 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	18414
Covered Lines:	16447
Line Coverage:	89.32%
Coverage Strength:	272788.8 hits per line

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💛 - Coveralls