Guarantee path-as-identity sites use NormalizedPath (map keys + Eq)

[zackees]

Narrow replacement for #1271 (closed — see the closing comment there for why the blanket PathBuf→NormalizedPath migration is the wrong shape).

The gap

fbuild's path-identity bug class (#436/#437/#282) is already covered by two operation-targeting lints:

• ban_raw_path_prefix_compare — starts_with/strip_prefix on raw paths (allowlist: 17)
• ban_manual_slash_normalize — hand-rolled \→/ (allowlist: 5)

The one hazard neither catches: a raw Path/PathBuf used as a HashMap/HashSet key or compared with ==. Raw Path equality is byte-exact, so C:\Foo ≠ c:/foo and //?/C:\x ≠ C:\x — exactly the Windows cache-key/watcher mismatch from #436/#437. NormalizedPath (case-folded, slash-normalized, UNC-stripped key) is the fix, and it is already used in the ~51 files that hold the primary keys.

Scope (bounded — do NOT expand to a type-wide ban)

• Audit sites where a path is a map/set key or an ==/match operand:
  rg 'HashMap<PathBuf|HashSet<PathBuf|BTreeMap<PathBuf|HashMap<.*&?Path' plus raw path_a == path_b comparisons in crates/*/src/.
• For each, confirm the key type is NormalizedPath (or the comparison routes through normalize_for_key). Migrate only these.
• Consider a focused dylint: "raw Path/PathBuf as HashMap/HashSet key" — the residual the existing two lints miss. Only if the audit finds enough live sites to justify it.

Explicit non-goals

• Not a blanket PathBuf→NormalizedPath swap. Plumbing paths (the ~3200 .join() + ~650 -> PathBuf that feed argv/Command) stay PathBuf.
• Not driving the ban_std_pathbuf allowlist to zero. Leave it as a documented legacy no-op (or delete it — the operation lints carry the real load).
• Do not swap physical canonicalization for NormalizedPath: NormalizedPath::new() strips .. lexically (path.rs:295) and case-folds key (path.rs:332), which is wrong across symlinks. library_manager::resolve_local_library_dir (test-emu does not resolve symlink library dependencies on Windows #1256/fix(library): resolve relative local dependency roots #1258) must keep std::fs::canonicalize.

Prerequisite if a migration reveals path-algebra needs

Only NormalizedPath::join() returns Self; parent()/with_extension()/components()/strip_prefix() deref to &Path/PathBuf. If the audit needs in-type path manipulation, add those Self-returning methods first — with a written decision on the lexical-.. / case-fold semantics — rather than sprinkling .into().

Related

#1271 (closed), #436, #437, #282, ban_raw_path_prefix_compare, ban_manual_slash_normalize.