Add specialized IntMap for structured serialization

[Frotty]

Summary

• add IntMap with direct integer keys and compiler-specialized typed value storage
• replace the structured reader's legacy HashMap and IterableMap pair with one typed field map
• make migration replacement and rename ownership explicit and correct
• cover primitive, class, tuple, replacement, removal, clear/reuse, and reader-mutation behavior

Design

Wurst currently rejects mixed legacy/specialized generic declarations, while JASS hashtables require integer child keys. IntMap is therefore the smallest backend-aligned abstraction: the key needs no adapter and values avoid legacy integer erasure and typecasting.

Verification

• grill typecheck --quiet
• grill test --quiet
• git diff --check

[Frotty]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: 43fd428de5

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