fix(hir): class field/element initializer test262 parity (v2, native-region-safe)

[proggeramlug]

Re-do of #4764 (correct on test262 but failed the compiler-output-regression / native-region-proof gate). This version keeps that gate green and scopes changes to class field/element semantics.

Results

• language/statements/class/elements + language/expressions/class/elements: 50.1% → 75.5% (+722 tests; 1420 → 2142 pass).
• native-region-proof gate: status: pass, NO failed_workloads on a debug build locally (the config CI uses). The class-field codegen change does not perturb the generic array/buffer native-region proofs.
• Zero real regressions: broad built-ins language shard 0/12 went 76.9% → 77.9% (+61 gains). The 40 apparent shard "regressions" each pass in isolation on both this branch and origin/main — parallel-run timeout flakiness (the shard ran while a build saturated the shared box), confirmed by re-running all 40 individually (isolated-pass=40, isolated-fail=0).

Root causes fixed

1. this undefined in static methods. C.m() binds this to the class; compile_static_method now seeds the this slot with the class-ref value and sets class_stack, so this.x / this.#x() / this[k] resolve.
2. Private (#x) elements leaked into reflection. Now hidden (gated on class_id != 0, so {"#fff":1} literals stay visible) from Object.keys/values/entries, getOwnPropertyNames, getOwnPropertyDescriptor, hasOwnProperty, JSON.stringify, and object spread / Object.assign — on instances and the class constructor (class-ref).
3. Static field/element initializers never ran for class EXPRESSIONS (var C = class { static x = 1 }). The var C = class {…} fast path and the general class-expression arm now emit inline StaticFieldSet/ClassStaticSymbolSet/static-block calls in source order (matching declarations); the per-evaluation ClassExprFresh path is reserved for class expressions inside function bodies (factories).
4. Dynamic get/set on a runtime class-ref VALUE returned/dropped silently (C[key] / C[key]=v where C is e.g. a function param — propertyHelper's isWritable(C, name)). js_dyn_index_get/set now handle the 0x7FFE tag; codegen preserves class-ref tag bits at runtime for runtime-unknown receivers (classref_preserving_handle); the array string-key setter guards class-ref receivers instead of dereferencing them as a GC header.

Files

• perry-codegen: codegen/method.rs, expr/index_get.rs, expr/index_set.rs
• perry-hir: lower/lower_expr.rs, lower/stmt.rs
• perry-runtime: object/{descriptors,object_ops,field_get_set,alloc}.rs, json/stringify.rs, value/dyn_index.rs, array/indexing.rs, typed_feedback.rs

Out of scope (separate feature areas; follow-up PRs)

• async-generator yield* / async-iterator delegation semantics (the largest remaining class/elements cluster, but a generator-runtime feature)
• parser-level grammar early-errors (invalid private-name SyntaxErrors)