fix: heap-allocate map/set structs to prevent class field segfaults

[cs01]

Summary

• Map/Set/StringMap/StringSet variable declarations used stack allocation (alloca) for the collection struct. When stored in class fields, the stack frame is freed after the constructor returns, leaving a dangling pointer — causing segfaults on later access.
• Changed all 5 collection allocation sites in variable-allocator.ts to use GC_malloc (heap allocation), matching the pattern already used by the literal generators in map.ts and set.ts.

Test plan

• New test fixture maps/map-class-field.ts — Map stored in class field, accessed after construction
• npm run verify:quick passes (tests + self-hosting Stage 1)

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