(1) class objects: foundation — class expressions allocate a heap class object with per-evaluation own static fields

[proggeramlug]

Part of #1785. Design: #1772.

(1) Foundation — class objects + per-evaluation own static fields

Scope

Lower a class expression (HIR Expr::ClassExprFresh) to allocate a real heap class object: js_object_alloc(template_class_id, n) (so class_id = the compile-time template), set the per-evaluation named static fields as own properties (js_object_set_field_by_name) and symbol static fields (js_object_set_symbol_property), and yield the object POINTER as the class value. Top-level class declarations are unchanged (stay INT32(class_id)).

Acceptance criteria

• function make(a){ return class { static x = a } } → make(1) !== make(2) and make(1).x === 1, make(2).x === 2.
• A no-this static method on such a class still dispatches and returns correctly (via class_id = template).
• Top-level class declarations + existing class test suite unchanged (no regression).
• cargo test (runtime/hir/codegen/transform) green; cargo fmt clean.

Status

WIP-complete on branch fix-1772-class-expr-identity (commits reworking Expr::ClassExprFresh lowering + the codegen in expr/static_field_meta.rs). Verified: distinct identity + distinct own static fields. Needs final cleanup (drop the dead integer-hack runtime helpers js_class_clone_for_expr + the CLASS_DYNAMIC_PROPS / symbol parent-walks added during the abandoned approach) and a node-validatable gap test before this lands as its own PR.

Blocks

(2), (3), (4), (5).