fix(codegen): uniform heap variant representation (fixes mixed-rep match)

[hyperpolymath]

Item 3b of the batch — the mixed-representation match half of #607. ⚠️ This changes the variant value ABI in the core-Wasm backend, so worth a careful review.

Bug

Zero-arg variants (None, plain enum cases) were emitted as raw i32 tags; arg-carrying variants (Some(x), Ok/Err) as heap pointers to [tag][fields]. A match over a value that can be either form (Option/Result) mis-read one as the other:

match None { Some(v) => v, None => d }   // returned garbage, not d

The Some(v) arm dereferenced the raw tag 1 as a pointer → wrong arm / garbage bind. (unwrap_or(None, 99) returned 0.)

Fix

Box zero-arg variants as a heap [tag] too, so every variant value is uniformly a pointer. The zero-arg match arm now dereferences [ptr+0] for the tag — symmetric with construction and with the existing args path. A small helper gen_box_zero_arg_variant is reused at the three construction sites (bare-ident ExprVar, ExprVariant ×2) and the zero-arg PatCon match (×2).

Verified (under node)

unwrap_or(None, 99)   = 99   ✅ (was 0)
unwrap_or(Some 7, 99) = 7    ✅
divide -> Ok/Err            ✅
plain enum Color{Red|Green|Blue} match/construct  ✅

• dune test 461 green — no regression from the ABI change.
• New runtime regression tests/codegen/mixed_variant_match (main()==9907), executed by run_codegen_wasm_tests.sh in the build job. (The .wasm is git-ignored and regenerated from the .affine by the harness, matching how the other fixtures run.)

Reviewer note

This is a representation change touching all variant construction + zero-arg matching. I validated the full suite + targeted runtime cases, but it's the kind of change worth a careful look. Trade-off: zero-arg variants now heap-allocate (4 bytes) like the args variants — consistent with the backend's existing bump-allocator model.

Closes the mixed-representation match item in #607.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Lz7pRcec2Z3tVtaAhvB3M8

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Generated by Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 43 issues detected

Severity	Count
🔴 Critical	2
🟠 High	25
🟡 Medium	16

⚠️ Action Required: Critical security issues found!

View findings

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