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List indexing/assignment and list multiplication across all four engines (#303)

* Spec list indexing/assignment and list multiplication across all four engines (#299, #294)

Adds the Python §3/§4 rules from #299 (index range -len..len-1 with negative
wraparound, bool rejected as an index, no auto-extend on assignment, and
`list * int`/`int * list` as a shallow-copying generic list constructor) to
docs/specs, and brings CSE, PVML, py2js, and WASM into conformance:

- CSE: fixed a broken negative-index read and a sign bug in negative-index
  assignment (JS `%` doesn't wrap like Python's), added bounds checking,
  distinct access/assignment IndexError wording, a dedicated
  "list indices must be integers" TypeError, and list multiplication.
- PVML: fixed #294 (silent auto-grow on out-of-range assignment), added
  negative wraparound, bool/type rejection, and list multiplication. List
  literal construction now pre-sizes the array with real bigint indices in
  browser-pathway compilation, while native-Pynter-targeted bytecode (which
  collapses int/float into one JS representation) keeps its original
  lenient semantics via a new legacyArraySemantics interpreter option.
- py2js: fixed the negative-index bugs that were deliberately kept
  bug-compatible with CSE, and added list multiplication.
- WASM: added negative-index wraparound, assignment-specific error wording,
  and list multiplication via a new LIST_REPEAT_FX runtime function.

Extends the shared operator-spec.ts conformance table (drives the CSE/PVML/
WASM/py2js sweeps) and adds directed test cases, including the shallow-copy
identity check (x = [[1,2]]*4; x[0] is x[1]) across all four engines.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FYRzwE7NXi2Tkf5wcSQgM4

* Fix list-repeat spread-argument RangeError risk and WASM count sign bug

Per gemini-code-assist review on #303:
- CSE/PVML/py2js: replace repeated.push(...list) with a nested... (continued)

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