labels: expose analyze() for corpus regression tooling

Promotes `mod labels` to `pub mod labels` and adds `analyze()` plus
`LabelAnalysis` (both `#[doc(hidden)]`) so an out-of-tree diagnostic
binary (freemkv-tools labels-analyze) can introspect which BD-J parser
matched a given disc, what JAR files the discriminators saw, and what
labels came out — without going through the production `apply()` path
that mutates DiscTitles.

Also adds `tracing::info!(parser = name, "label parser matched")` /
"no label parser matched" inside `extract()`. Dev-only signal: users
get the same seamless behavior; developers can finally tell whether a
disc hit a real parser or fell through to the codec-name fallback in
fill_defaults().

The new `jar_inventory()` helper deduplicates and sorts filenames
under any `/BDMV/JAR/<x>/` subdirectory — same plumbing the existing
`jar_file_exists()` discriminators use, just enumerated rather than
predicate-tested. Used by `analyze()` to surface unrecognized
parser-source files when no parser matches, which is the input to
"do we need a new parser?" triage.

No behavior change to the production label path. `apply()` and
`extract()` remain functionally identical; the new public surface
exists alongside.

src/lib.rs

@@ -84,7 +84,7 @@ pub(crate) mod identity;
 pub(crate) mod ifo;
 pub(crate) mod io;
 pub mod keydb;
-pub(crate) mod labels;
+pub mod labels;
 pub(crate) mod mpls;
 pub mod mux;
 pub mod pes;