fix(subtitles): collapse duplicate ASS events and decode text once

- Drop vector drawing runs (\p1...\p0) and collapse duplicate/animation-burst cue events (karaoke frames, layered shadow copies) so typeset ASS scripts stop flooding the subtitle sidebar
- Add ass-text.ts to decode ASS text once at ingestion, mirroring mpv's ass_to_plaintext (\N/\n/\h handling, unclosed braces, no \{ \} \\ escapes)
- Renderer, timing tracker, tokenizer, and tokenization cache now consume the already-decoded plain text instead of each re-deriving it, fixing mismatched cache keys for the same authored line

changes/styled-subtitle-cue-parsing.md

@@ -0,0 +1,5 @@
+type: fixed
+area: subtitles
+
+- Heavily typeset ASS scripts (karaoke OP/ED, sign work) no longer fill the subtitle sidebar with garbage. Vector drawing runs (`\p1` … `\p0`) are no longer shown as subtitle text (e.g. `m 20 0 b 10 0 0 10 0 20 …`), and duplicate events in a parsed subtitle file collapse into one cue: identical text over an identical span (layered "shadow" copies), and per-frame animation bursts. An ASS burst has to prove itself with authoring evidence — a temporal tag (`\t`, `\move`, karaoke timing), an animated `Effect` column, or override values that change from event to event — plus one shared style and actor, so three rapid `えっ` reactions from three characters, or a sign repeated with the same static `\clip`, stay separate. SRT and VTT carry no such metadata, so there the run has to be at least five contiguous events all shorter than 0.1s, which is where ASS-to-SRT conversion leaves karaoke frames.
+- Subtitle text is now decoded from ASS exactly once, where it enters the app, and matches how mpv renders the same line (including `\N`, `\n`, `\h`, unclosed `{`, and the fact that `\{` is not an escape). Renderer, timing tracker, tokenizer and the tokenization cache take that decoded text as-is instead of each re-deriving it, so one authored line can no longer produce two different cache keys, and a cue that normalizes to nothing is no longer stored as subtitle text or cached under an empty key.