Per-file module naming

The theoremc::mangle module provides per-file module naming for .theorem file paths. Each path is transformed into a deterministic, collision-resistant Rust module name of the form:

__theoremc__file__{path_mangle(path_stem(P))}__{hash12(P)}

• path_stem(path) -> PathStem — removes a trailing .theorem extension if present; otherwise returns path unchanged.
• path_mangle(&PathStem) — sanitizes a path stem into a Rust-identifier-safe fragment using the five-step algorithm from docs/name-mangling-rules.md §1: 1. Replace / and \ with __. 2. Replace any character not in [A-Za-z0-9_] with _. 3. Collapse consecutive _ to a single _. 4. Lowercase the result. 5. If the result starts with a digit, prefix _.
• hash12(path) — computes the first 12 lowercase hex characters of the blake3 digest of the original path string (not the mangled stem).

mangle_module_path(path) combines the building blocks and returns a MangledModule struct with accessors:

use theoremc::mangle::mangle_module_path;

let m = mangle_module_path("theorems/bidirectional.theorem");
assert_eq!(m.stem(), "theorems/bidirectional");
assert_eq!(m.mangled_stem(), "theorems_bidirectional");
assert_eq!(m.hash(), "1fc14bdf614f");
assert_eq!(
    m.module_name(),
    "__theoremc__file__theorems_bidirectional__1fc14bdf614f",
);

Paths that differ only in characters lost during sanitization (e.g., theorems/my-file.theorem and theorems/my_file.theorem) produce the same mangled stem but different module names because hash12 operates on the original path string. The 12-character blake3 hash suffix provides the real disambiguator.