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py2js: compile-to-JavaScript evaluator for Python §1-4 (#282)

* Add py2js engine: compile-to-JavaScript evaluator for SICPy chapter 1

A fourth execution engine (alongside CSE, PVML and WASM) that compiles
the py-slang AST to JavaScript source and runs it natively, in the style
of js-slang's transpiler:

- src/engines/py2js/runtime.ts: unboxed value model (int=bigint,
  float=number, None=null, complex=PyComplexNumber objects); operator
  helpers mirror the CSE machine's chapter-1 semantics (same dispatch
  order, same §1 restrictions); tail-call markers + trampoline for
  proper tail calls; dual compilation support (def2/acall/callSync) so
  conductor modules can call user functions synchronously in hot loops
  while the program spine stays async-capable.
- src/engines/py2js/compiler.ts: AST-to-JS codegen, $-prefix identifier
  mangling, sync and dual modes; exec-style only (no last-expression
  value, matching Python semantics).
- src/engines/py2js/index.ts: runCodePy2Js/runCodePy2JsDual — parse,
  resolve with the chapter validators, compile, run; chapter 1 only.
- src/tests/operator-conformance-py2js.test.ts: table-driven operator
  conformance sweep (749 cases) over the docs/specs typing tables,
  pinned against a freshly computed CSE reference with print()-wrapped
  observation, mirroring the PVML-in-browser sweep.
- src/types/value-types.ts: extract context-free PyComplexNumber.divBy
  so py2js reuses the CPython-style division algorithm without a CSE
  Context.
- experiments/py2js/: the exploratory prototype, differential harness
  and benchmarks that motivated the design (CSE/PVML comparisons,
  dual-compilation sound-module measurements); not wired into the
  build or tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FbExZBaxgeUL4oi1t7PG93

* Address #282 Gemini review: hypot for complex abs; guard non-finite math_floor; keep error kind in Py2JsRunError

- abs(complex) uses Math.... (continued)

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