funcache coverage, tests, and comments

[aqk]

Note

Medium Risk
Mostly adds/extends tests and documentation, but also changes @ two-argument codegen behavior and error reporting, which can affect program compilation/runtime for env-path references.

Overview
Improves Funcache clarity and coverage by documenting what get_function_cache_key hashes (and explicitly what it does not), and adding tests that lock down cache-key invariants across dialects plus cold-vs-warm determinism.

Expands module disk-cache test coverage to compile real modules twice to assert cache hits/misses (including after source edits), adds determinism and cyclic-constant deadlock regression tests, and adds optimizer tests for stepping_over_24.

Adjusts codegen handling for the two-argument (@ X N) form (including applied-path cases) and refines the strict-mode error message; also fixes a small CompilerOpts::get_file_mod_date parameter typo.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Per-helper warm-cache assertion is tautological

Low Severity

The per-helper comparison loop in funcache_cold_vs_warm_roundtrip iterates cold_cache and looks up each key in warm_cache, but warm_cache was initialized directly from cold_cache.clone() and the codegen path only inserts on cache miss. As a result, every entry compared is the very same Rc<SExp> placed there before warm codegen ran, so this loop cannot fail and adds no coverage beyond the overall-program assertion above it.

 

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