self-host: Int.div + float-literal interpolation lexing + replay Unit normalization

[jasisz]

A VM-vs-self-host parity audit during 0.24 prep found the self-hosted interpreter lagged the other backends. This PR closes the three tractable gaps; the two larger ones (higher-order functions, multi-module type resolution) are tracked in #436.

Int.div on self-host

The interpreter's builtin registry knew Int.mod but not Int.div, so any program using the 0.24 Int.div idiom hit unknown int builtin: Int.div. Added builtinIntDiv, delegating to the real Int.div so it inherits full partiality (division by zero + overflow) and the canonical error strings.

Float literals in string interpolation

"{1.5}" lexed as TkInt then choked on the . — the interpolation lexer (tokenizeInterpDigit) always emitted an int and never checked for a decimal point. Mirrored the main-path float handling with interpolation-aware variants. (Float literals outside interpolation already worked.)

Replay Unit normalization (all rust/self-host targets)

The VM and wasm-gc replay formats serialize a Unit return as JSON null; the rust/self-host backend wrapped it as a ValUnit variant object, so a recording made by one backend spuriously failed to replay on the self-host (mismatch → panic) for any Unit-returning main. The --with-replay codegen now canonicalizes ValUnit back to null at the record/replay serialization points.

Result

• Non-interactive example corpus, aver run vs aver run --self-host: 17/25 → 21/25 parity (remaining 4 are the multi-module/refinement cases in self-host parity: higher-order functions + multi-module type resolution #436).
• A 0.23-recorded wumpus game session (116 effects) now replays MATCH on the self-host — matching VM and wasm-gc, so the cross-version replay safety net holds on every backend.
• src/self_host/ regenerated via aver compile --target rust.

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