feat: add repl precompiler opt-in and docs hardening

[rlippmann]

What changed:

• Added opt-in REPL flag --with-precompiler with heuristic + validation preprocessing in the REPL loop.
• Preserved default REPL behavior when the flag is not provided.
• Added REPL tests for flag behavior, strict CLI argument handling, and subprocess stdin smoke behavior.
• Updated user-facing docs and integration docs for 0.6.9 to reflect reject-first precompiler behavior.
• Tightened experimental preprocessor contract docs (directive / no_directive / unknown, reject-first near-miss handling, no grammar expansion).

Why this change was needed:

• 0.6.9 introduces explicit REPL opt-in preprocessing while keeping engine semantics unchanged.
• Documentation needed to be aligned with current hardened precompiler contract so near-miss behavior is described accurately.