Open enums (Strategy B): per-case method overrides, hierarchical subt…

[eliotmoss]

…ypes, fields, methods, squahed

Squashed snapshot of open_enums3b past master. Adds:

• Open enum types with hierarchical subtypes (enum E.More { ... }), case _ defaults, DFS-assigned tag ranges for subtype matching, and qualified case access (E.More.C).
• Per-case enum method overrides (Case { def m() ... }) via synthetic per-case VstClass / IrClass / RaClass — Strategy B. Cases without overrides share the parent RaClass (elision).
• Subtype enum method overrides (enum E.More { ...; def m() ... }) with virtual dispatch through the root enum's mtable.
• Subtype field redeclaration / addition (E.S(super, f: float), E.S(super), E.S { D(15) }) with multi-level inheritance.
• EnumSetType for hierarchies (one set type per root, bit positions by DFS tag).
• Enum method closures with proper boxing on JVM (Integer) and wasm-gc (i31ref); Oop/raw tag receiver convention switched per target via NormConfig.
• Queue-based two-dimensional liveness (live cases x live virtuals -> live impls) and -compact-mtable=N optimization for compacting redundant mtable rows via tag->slot indirection.
• JvmV3EnumGen M_ABSTRACT guard, SsaInliner enum NullCheck skip, and other backend fixes for v3i / x86 / x86-64 / jar / wasm / wasm-gc.
• One master regression fix (lookupEnumExprMember VstField receiver), benchmarks (EnumScalability, EnumLiveness, EnumOverrides, EnumClosureBoxing), and the wasm bench runner.
• Tutorial and grammar reference updates for open enums.

37/37 ci/Makefile configs pass 3809/3809 tests.