v0.20.1: delete SectorReader, extract Disc::patch, doc/stub cleanup

WO-2 (delete SectorReader trait):
- The 0.18 trait split into SectorSource (read-only) and SectorSink
  (write-only) is final; the legacy SectorReader alias was a bridge.
- Renames every internal &mut dyn SectorReader (~25 sites) to
  &mut dyn SectorSource. The trait method capacity() becomes
  capacity_sectors() with a default of 0 (preserves SectorReader's
  default-0 behavior).
- Deletes the SectorReader trait, its blanket-to-Source bridge, and
  the FileSectorReader type alias. Adds explicit forwarding impls
  for Box<dyn SectorSource> and &mut dyn SectorSource so generic
  decorators like DecryptingSectorSource<S: SectorSource> compose.

WO-3a (extract Disc::patch):
- Moves Disc::patch (1230 lines) and bytes_bad_in_title from
  disc/mod.rs into disc/patch.rs as a split inherent impl. Zero
  behavior change — pure mechanical relocation. disc/mod.rs drops
  from 3,945 to 2,714 LOC.

WO-6 (partial):
- Deletes src/labels/png_filenames.rs — was a 72-LOC stub with
  detect() returning false, never wired into the PARSERS registry.

project docs doc drift fixes (audited 2026-05-13):
- JUMP_BASE_SECTORS: 256→1024 (64 MB base for UHD, not 8 MB)
- PASSN_DAMAGE_THRESHOLD_PCT: 12→6
- PASSN_SKIP_SECTORS_BASE: 64→32
- MAX_RANGE_SECS=180: replaced by proportional range_sectors × 25,
  capped at RANGE_BUDGET_CAP_SECS=1800.

src/labels/png_filenames.rs

@@ -1,72 +0,0 @@
-//! PNG-filename language token parser — stubbed (noop) pending need.
-//!
-//! ## What this would do
-//!
-//! Some discs encode per-language menu localization as pre-rendered PNG
-//! menu buttons, one per language, with the language token embedded in
-//! the filename. Examples observed in the 2026-05-10 corpus:
-//!
-//! - **disc-01 (The Amateur)** — `<region>_<lang>_<context>_<format>.png`
-//!   Region prefix: `USA` / `UK` / `JPN` / etc.
-//!   Lang tokens (3-char, uppercase): `ENG`, `FRC`, `FRP`, `DEU`, `DUT`,
-//!   `ITA`, `JPN`, `LAS`, `CSP`, `POL`, `CZE` (11 languages)
-//!
-//! - **disc-09 (Dune orig)** — `<title>_<variant>_<lang>_Composite<N>.png`
-//!   Lang tokens (3-char, mixed-case): `Eng`, `Ger` (2 languages)
-//!
-//! ## Why stubbed
-//!
-//! MPLS already gives per-stream `language` + `coding_type` + stream-type
-//! (audio vs subtitle) on every disc. For the 2 unknown-framework discs
-//! that PNG filenames would close (disc-01, disc-09), MPLS will produce
-//! a strict superset of what filenames could give us, because MPLS knows
-//! per-stream attribution while filenames only know "the disc offers
-//! these N language buttons."
-//!
-//! The **only** thing PNG filenames give us that MPLS doesn't is **studio
-//! variant disambiguation**:
-//! - `FRC` (French Canadian) vs `FRP` (French Parisian) — MPLS just says `fra`
-//! - `LAS` (Latin American Spanish) vs `CSP` (Castilian Spanish) — MPLS just says `spa`
-//!
-//! That's niche enough that it doesn't justify implementing right now.
-//! Reactivate this parser only when:
-//! 1. We hit a disc where MPLS is malformed/empty AND PNG filenames are
-//!    the only language hint, OR
-//! 2. A downstream consumer needs the studio variant suffix for output
-//!    naming (e.g. `Title (French Canadian).mkv` vs `Title (French).mkv`).
-//!
-//! ## When reactivating
-//!
-//! Implement `parse` to:
-//! 1. Iterate top-level PNG paths in `/BDMV/JAR/` (and `<id>/` subdirs).
-//! 2. Tokenize each filename on `_` / `-` / `.`
-//! 3. Match each token against an alias table:
-//!    - ISO 639-1 / 639-2 standard codes
-//!    - Studio variants: `FRC`/`FRP` → `fra-CA`/`fra-FR`,
-//!      `LAS`/`CSP` → `spa-419`/`spa-ES`,
-//!      mixed-case shortforms `Eng`/`Ger`/`Fra`/`Spa`/`Jpn` → ISO 639-2
-//!    - Country prefix filter: drop `USA`/`UK`/`JPN`/`AUS`/`GER`/`FR` when
-//!      they appear in position 0 (those are region markers, not langs).
-//! 4. Deduplicate. Confidence stays `Low` because we still don't know
-//!    per-stream codec or audio/subtitle attribution.
-//!
-//! Wire as ENRICHMENT after MPLS in `mod.rs::analyze`, not as a primary
-//! parser: PNG filenames upgrade `lang=fra` to `lang=fra-CA` when both
-//! sources agree on the disc; they should never overwrite MPLS data.
-
-use super::ParseResult;
-use crate::sector::SectorReader;
-use crate::udf::UdfFs;
-
-/// Stub: returns false so the dispatcher never calls `parse`. Reactivate
-/// by checking for the patterns described in the module docs.
-#[allow(dead_code)] // module-level noop, not wired into PARSERS until needed
-pub fn detect(_udf: &UdfFs) -> bool {
-    false
-}
-
-/// Stub: returns None. See module docs for the implementation sketch.
-#[allow(dead_code)] // module-level noop, not wired into PARSERS until needed
-pub fn parse(_reader: &mut dyn SectorReader, _udf: &UdfFs) -> Option<ParseResult> {
-    None
-}