labels: append CLPI orphan streams after gap-fill

Three layered sources of stream labels now, in precedence order:
1. **Framework parser** (paramount/criterion/pixelogic/ctrm/dbp/deluxe)
   — editorial labels with purpose/qualifier ("English Atmos",
   "Director's Commentary", "English SDH"). High or Medium confidence.
2. **MPLS gap-fill** (`fill_gaps_from_mpls`) — every stream the
   playlist references gets at least a basic lang+codec label, even
   when the framework parser missed it.
3. **CLPI orphan append** (`append_clpi_orphans`) — streams in
   /BDMV/CLIPINF/*.clpi ProgramInfo that NO MPLS playlist references.
   Empirical (2026-05-11): ~5% of streams across the 11-disc corpus,
   most dramatic on disc-02 (HDMV-only) at 40% CLPI-only.

Orphan numbering: each appended orphan gets
`stream_number = max(existing per type) + N` so playlist-reachable
streams keep their original positions and orphans sort cleanly at
the tail.

Orphan dedup: (stream_type, language, codec_hint) tuple — fuzzier
than PID matching (PIDs aren't carried on StreamLabel) but it's the
only signal available downstream of the gap-fill. False positives
(genuine orphan that happens to share lang+codec with an existing
entry) silently drop, which is the conservative failure mode — the
user-facing display would just see a confusing duplicate otherwise.

`mpls_universal::language_display_name` and `::codec_name` promoted
from private fn to pub(crate) so this module can build orphan labels
with consistent naming.

Tests: 2 new in gap_fill_tests — synthetic-input verification of the
dedup tuple logic and the stream_number assignment. 6/6 tests in the
gap-fill module now passing.

src/labels/mpls_universal.rs

@@ -175,7 +175,7 @@ fn normalize_language(raw: &str) -> String {
 /// Human-readable English name for an ISO 639-2 code, or empty if
 /// the code is unknown. Kept inline rather than in vocab because
 /// vocab is the *reverse* mapping (name → code).
-fn language_display_name(iso: &str) -> String {
+pub(crate) fn language_display_name(iso: &str) -> String {
     match iso {
         "eng" => "English",
         "fra" | "fre" => "French",
@@ -226,7 +226,7 @@ fn language_display_name(iso: &str) -> String {
 /// Map BD coding_type byte → codec name. Returns empty for unknown
 /// bytes (the table covers everything the spec defines, but unknown
 /// values are still possible on malformed discs).
-fn codec_name(coding_type: u8) -> &'static str {
+pub(crate) fn codec_name(coding_type: u8) -> &'static str {
     match coding_type {
         0x02 => "MPEG-2",
         0x1B => "H.264",