TypedPython: replace '# typedpython: parallel' comment with @compiled(parallel=True) #358

Per maintainer decision (2026-10-30), @compiled is the only compilation mark. Replace the # typedpython: parallel comment marker for SPEC N-15 parallel loops with @compiled(parallel=True) and remove all comment-marker recognition (PARALLEL_MARKER, _parallel_marked).

Scope:

• Update docs/SPEC.md (L-8, N-8, N-15) and docs/design/typedpython-parallel-loops.md (decision 5).
• Make builtins.compiled (.pth shim, Python3.installBuiltinCompiled in python-multiplatform, __builtins__.pyi stub, __main__.py, and test COMPILED_SHIM) accept both @compiled (compiled(f)) and @compiled(**opts) (compiled(**opts)(f)), aligning with the @compiled(fastmath=True) shape (TypedPython: per-function fastmath opt-in (@compiled(fastmath=True)), not CPython-identical #332).
• Update typedpython/frontend.py to recognise @compiled and @compiled(parallel=True) / @compiled(parallel=False), reject unknown or non-literal arguments with a clear skip reason, and drop # typedpython: parallel comment parsing.
• Convert tests (test_parallel_loops.py, test_builtin_compiled.py, test_frontend.py, BuiltinCompiledTest.kt) and benchmark helpers.

Done when: test_builtin_compiled.py, test_frontend.py, test_parallel_loops.py, :python-multiplatform:desktopTest --tests python.multiplatform.ffi.BuiltinCompiledTest, and the TypedPython suite pass.