fix(codegen): register struct_layouts for record-type aliases

[hyperpolymath]

Summary

Record-type aliases (type X = { ... }) silently miscompiled when used as function parameters or return types: every field access on such a value resolved to offset 0.

Root cause

type X = { a: T, b: U } parses to TopType { td_body = TyAlias (TyRecord (rfs, _)) } (parser.mly:431). The TopType branch in gen_decl (codegen.ml:2439) handled the analogous TyStruct case by registering a 4-byte-stride layout in ctx.struct_layouts, but the TyAlias branch caught everything with TyAlias _ -> Ok ctx. With no layout registered, every .field_N access fell back to offset 0 — a silent runtime miscompile, not a validation error or compile failure.

The fix adds a TyAlias (TyRecord (rfs, _)) branch immediately before the catch-all, mirroring the TyStruct case but unpacking the alias.

Regression coverage

Two new tests in the E2E WASM group:

• record-alias registers struct_layouts — parses type State = { health: Int, score: Int };, calls gen_decl directly, asserts (\"State\", [(\"health\", 0); (\"score\", 4)]) appears in ctx.struct_layouts. Verified to fail on main without the fix (stash-revert reproduces expected Some [...] but got None); passes with it.
• non-record alias leaves struct_layouts alone — guards against accidental over-broadening: type Plain = Int must still hit the catch-all and add no layout entry.

Verification

```
$ dune build && dune runtest
…
Test Successful in 0.082s. 329 tests run.
```

Was 327, now 329 with the two new tests. Full suite green.

Discovered during

Field trial taking airborne-submarine-squadron (a 29-field state record stepped at 60 fps over WASM) from "compiles" to "runs to conclusion". The product currently ships via the linear backend with flat-record boundaries; this fix removes the flatness constraint at the per-record-alias level. WasmGC-backend record/tuple layout has a separate, independent defect — out of scope here.

Test plan

• dune build clean
• dune runtest → 329/329 green
• New regression test verified to fail on main without the fix
• Catch-all TyAlias _ still reached for non-record aliases
• CI green

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 43 issues detected

Severity	Count
🔴 Critical	15
🟠 High	16
🟡 Medium	12

⚠️ Action Required: Critical security issues found!

View findings

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