Compiler: lower plain-function destructuring assignments natively

Parents: #465, #498 Related: #706, #724

Problem

Compiler.compileExpr rejects every array/object destructuring assignment when self.scope != null, so an otherwise eligible plain function or method falls back wholesale:

.arr_pattern, .obj_pattern => {
    if (self.scope != null) return error.Unsupported;
    ...
}

Generator/async environment mode already owns a bytecode-native pattern compiler for suspended defaults/computed keys. #706 generalized its temporaries to activation-local scratch for programs, and #724 expanded object-rest assignment targets. Frame mode can use the same native path with frame-local ActivationTemp values; retaining the bind_pattern tree-walker callback is not a valid completion because it cannot write statically resolved frame/upvalue slots.

Scope

• Lower array/object destructuring assignment in plain functions and methods through the existing native pattern compiler.
• Preserve RHS result identity, nested patterns, holes/rest, defaults, computed keys, identifier/member/super targets, anonymous-function naming, and one-time Reference evaluation.
• Preserve IteratorDestructuringAssignmentEvaluation ordering, IteratorClose on normal early completion and abrupt completion, RequireObjectCoercible, ToPropertyKey, CopyDataProperties exclusion order, and PutValue timing.
• Keep dynamic with/binding cases fail-closed when an activation-owned resolved binding reference is still required.
• Admit the same construct through program, plain, generator, async, and async-generator modes without routing a bytecode instruction back into the tree-walker.
• Keep native tiers fail-closed until their root maps and side exits model every new opcode/state they execute.

No-workaround rules

• No synthesized globals, AST-identity caches, duplicated RHS/key/default evaluation, relaxed IteratorClose, environment-only shadow writes, or hidden tree-walker callback inside an admitted bytecode chunk.
• No stacks, queues, jobs, Promise, microtask, Worker-lifecycle, or Map/Set iterator changes.

Acceptance

• Compiler tests prove plain frame/upvalue/global targets use native pattern lowering and no bind_pattern.
• Forced tree-walker ↔ required-bytecode differentials cover nested array/object/rest/default/computed/member/super targets and abrupt iterator closure.
• Generator/async suspension, moving GC, debugger/watchdog, and TSan witnesses preserve exact activation-local References.
• Exact nonzero Test262 destructuring-assignment before → after accounting has zero regressions.
• Focused compiler/frontend, full sharded units, and applicable sanitizer gates pass.

Implemented and landed in 525aa677 (feat(compiler): lower native destructuring assignments (#726)).\n\nPlain functions and methods now lower array/object assignment patterns entirely in bytecode, writing exact frame, upvalue, and global targets without bind_pattern. Member/super references remain one-shot, iterator next is cached once, IteratorStepValue done state and completion-aware IteratorClose preserve abrupt semantics, and object_rest has an exact rooted optimizer side exit. Identifier targets inside with remain fail-closed where observable HasBinding ordering still needs an activation-owned resolved binding reference.\n\nValidation:\n\n- focused runtime differential: 1 passed, 0 failed/leaked across five tree-walker/required-bytecode witnesses;\n- compiler opcode and optimizer root-map gates: 2 passed;\n- focused TSan: 1 passed, 0 races/failures/leaks;\n- focused frontend: 13/13;\n- full units: 1,852 passed, 1 platform skip, 0 failed, 0 leaked across 10 shards;\n- exact Test262 test/language/expressions/assignment/dstr: 0 fail / 368 pass -> 0 fail / 368 pass, byte-identical transcripts.\n\nClosing.

#736 is complete in e2ba5ac3 and reuses this issue’s native BindingInitialization machinery directly for initializer-free formal patterns. The admitted bytecode contains iterator/object-rest/frame stores and no bind_pattern or tree-walker callback.