WebAssembly Core 2.0: implement multi-value functions and control flow

[chrisbbreuer]

Parent: #263
Depends on: #262
Roadmap: #142, #134

Implement multi-value end to end without retaining MVP-only single-result shortcuts.

Acceptance:

• Decode and validate multi-result function types and type-index block signatures.
• Generalize validator control frames, branches, calls, returns, and unreachable polymorphism to ordered result vectors.
• Generalize interpreter labels, calls, returns, host imports, and exported JavaScript functions to ordered result vectors.
• Implement JavaScript API multi-value array results and cross-instance/import behavior.
• Add focused binary, validator, executor, and API coverage with stable disabled-feature diagnostics.

[chrisbbreuer]

Implemented and verified on main.

Evidence:

• Multi-result signatures, type-index blocks, generalized validator/control/runtime, export arrays, and iterable imports: fcae6362
• Loop parameters, vector branches, exact import arity, and cross-instance behavior: 0abf692c
• br, br_if, br_table, calls, implicit else, and cross-instance linking: 53a39b0c
• Explicit multi-result return and registry ownership: 27eafb9d
• Published status and benchmark applicability: a9e617e5
• Complete focused WebAssembly suite: 141/141 passed, 0 skipped, 0 failed, 0 leaked
• Feature-registry drift verifier: 4 profiles / 12 pinned features

The structural parent #263 remains open for reference types (#271), bulk memory (#272), and the exact upstream score (#273).