Add behavioral TS parity tests for runtime core FFI surface

[TheHypnoo]

Follow-up to #694.

Goal

Convert the runtime core FFI surface inventory into deterministic behavioral TypeScript parity tests.

Scope

Start from test-files/test_ffi_surface_runtime_core.ts and move coverage entries into focused fixtures that execute real behavior against Perry and Node where possible.

Suggested areas:

• arrays and iterators
• objects, descriptors, symbols, and prototype behavior
• strings, regex, JSON, URL, Date, Math, and numeric coercion
• promises, async helpers, errors, exceptions, and timers
• buffers, typed arrays, values, GC-sensitive paths, and native dispatch helpers

Acceptance criteria

• New or expanded .ts fixtures under test-files/ exercise deterministic behavior.
• Fixtures pass through ./run_parity_tests.sh --filter <name> or use expected-output files when Node is not the right oracle.
• Entries converted to real tests are removed from test_ffi_surface_runtime_core.ts by running ./test-coverage/regen_ts_surface_inventory.py.
• ./test-coverage/audit.sh --markdown still reports full TypeScript FFI surface coverage.