Carry a block's next value through flat_map

[ryanseys]

Summary

flat_map evaluated the block's array as a plain tail expression, so a next [..] inside the block was lowered to a bare continue and lost.

[1, 2, 3].flat_map { |x| next [0] if x == 2; [x, x] }
# was: [1, 1, 3, 3]      (the `next [0]` dropped)
# now: [1, 1, 0, 3, 3]   (matches CRuby)

This mirrors the array map/select/reject paths, which already route the block value through the next-aware helper.

Implementation

Fill the per-iteration array temp through the shared emit_block_value_into helper (initialized to its type default) before splatting its elements into the result — so a next [..] flattens its array exactly like the tail expression does. The forward declaration of emit_block_value_into moves to the top of the file so emit_flat_map_expr (above the definition) can see it.

Testing

test/next_value_flat_map.rb covers next [..] in flat_map over int and string element arrays, plus a plain (no-next) flat_map to pin the unchanged path. Expected regenerated from ruby. Verified under ASAN+UBSAN with SPINEL_GC_STRESS=1, matching ruby.

[gemini-code-assist]

Code Review

This pull request updates the code generator in src/codegen_fold.c to correctly handle next statements with array values inside flat_map blocks. Specifically, it introduces a forward declaration for emit_block_value_into and uses it within emit_flat_map_expr to collect the block's next-aware value into a temporary array variable, ensuring that next [..] flattens its array like the tail expression. It also adds corresponding test cases and expected outputs. There are no review comments, so I have no feedback to provide.

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