[proggeramlug]

Summary

Operator-expression parity across language/expressions/* (the 8 assigned dirs):
617→669 pass (82.6%→89.6%), with 0 regressions on the broad built-ins language
shard 0/12 (and +6 bonus fixes in equals/ and strict-does-not-equals/ from the
BigInt equality work).

Per-dir (rt-fail before→after): does-not-equals 13→0; postfix-increment 14→4;
postfix-decrement 14→4; prefix-increment 13→4; prefix-decrement 13→4; instanceof 11→10;
compound-assignment 33→33 (deferred); logical-assignment 3→3 (deferred).

Root causes fixed

1. inc/dec ToNumber (expr/literals_vars.rs, ~30 tests). x++/--x did a raw
   fadd on the loaded NaN-boxed bits and returned the original operand, so
   var x=true; x++ left x as true. The operand is now ToNumber-coerced
   (js_number_coerce) before the step and the postfix result is the coerced number.
   Gated to skip statically-integer loop counters so the hot path stays a single fadd.

2. i32-bounded Update gating (collectors/i32_locals.rs, ~8 tests).
   is_strictly_i32_bounded_expr treated any Expr::Update as i32-bounded, so
   var y = x++ got an i32 slot and truncated the postfix result
   (var x=1.1; var y=x++ stored y=1). Now i32-bounded only when the updated local
   is itself a known integer.

3. !=/== with BigInt and objects (expr/compare.rs + builtins/arithmetic.rs,
   does-not-equals 13→0). The codegen BigInt fast path fired when either side was
   BigInt and fed js_bigint_cmp a non-BigInt operand (0n != undefined dereferenced
   an undefined NaN-box as a BigIntHeader); narrowed to both-sides BigInt. Added full
   BigInt abstract equality (BigInt↔BigInt/Number/String/Boolean), Object→ToPrimitive,
   and Object-vs-Object reference equality (new Boolean(true) != new Boolean(true)) to
   js_loose_eq.

4. instanceof non-constructor RHS (lower/lower_expr.rs). A primitive/this/call
   right-hand side was silently rewritten to instanceof Object and returned false;
   it is now lowered to a value and routed through js_instanceof_dynamic, which throws
   the spec TypeError (true instanceof true, ({}) instanceof this).

Deferred (noted in code)

• compound-assignment A7 left-before-right single-evaluation (needs expression-level
  temp-slot machinery in the shared expr_assign.rs).
• instanceof OrdinaryHasInstance .prototype-must-be-object + undefined-ident ReferenceError.
• BigInt ++/-- strict-eq (runtime value is already correct BigInt, but === uses
  fcmp because the Update-result static type is inferred as Number).
• with/eval/global compound + update compile-fails.

Validation

• Build clean; cargo fmt --all applied; file-size gate passes.
• Zero regressions verified by diffing failure sets of branch vs a from-scratch
  origin/main build over --dir built-ins language --shard 0/12 (2645 judged).