migrate(C2 wave 2a): VmMulDiv scalar multiply/divide, 4-gate green

[hyperpolymath]

C2 wave 2a — scalar multiply/divide

Mul and Div turned out to be pure scalar value-transforms, not memory opcodes, so they need no array/linear-memory ABI — split out of "C2 wave 2" and landed ahead of the memory-bound wave 2b.

VmMulDiv (11 exports) re-decomposed per the brain/senses split — the register-name dict stays host-side, the brain is the scalar-int value transform. Reversibility migrated as headline exports:

• mul_roundtrip — Bennett ancilla (c := c + a*b, inverse c := c - a*b), invert∘execute == id.
• div_reconstruct — q*b + r == a for all b (incl. divide-by-zero, where r holds the original a).

The intentional-flaw in-place/simple variants are migrated as value transforms with no roundtrip==id claim (the source ships them to demonstrate where in-place multiply / simple divide break reversibility).

Four gates green

Gate	Result
G1 compile	1/1 (WASM, validates; 11 exports)
G2 parity	3322/3322 (mul reversibility + div reconstruction + every guard)
G3 boundary	n/a (numeric transform — no value↔integer encoding table)
G4 assail	1/1 clean

Two new i32 ABI facts surfaced and handled

1. JS a*b loses precision above 2^53 → the parity oracle uses Math.imul to stay i32-exact against i32.mul (a naive (a*b)|0 oracle would mismatch at the domain extremes, e.g. INT_MAX*INT_MAX).
2. i32.div_s TRAPS on b==0 and INT_MIN/-1 (ReScript wraps the latter), and i32.rem_s TRAPS on b==0 → the brain guards both (nested-if, avoiding the unverified && codegen path) so the wasm is a total function matching ReScript's observable output — never a trap.

Ledger + migration-map.json updated: C2 wave 2a DONE; wave 2b (memory/control opcodes + structural VM) correctly scoped as needing the array/linear-memory ABI (the next real unblocker, also feeds the string wall).

Staged-only under proposals/idaptik/migrated/ — idaptik write-gate honoured.

https://claude.ai/code/session_01WoKhFQePiRsAj7aqnxbG8s

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[github-actions]

🔍 Hypatia Security Scan

Findings: 87 issues detected

Severity	Count
🔴 Critical	2
🟠 High	15
🟡 Medium	70

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