Fix/parser and search perf#23
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abdelaziz-mahdy wants to merge 3 commits intodsc-uob:masterfrom
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Subtitle load failures used to vanish into zone error handlers — the provider fire-and-forgets `initial()`, so a bad URL, non-200, or unsupported extension produced silent "no subs rendered" with no breadcrumbs. Now every fetch logs url/scheme/host/extension/header-keys, every response logs status + body head, every exception logs with stack, every zero-match parse logs body head + detected type, and UnsupportedSubtitleFormat logs the rejected extension. Uses `dart:developer.log(name: 'subtitle')` so nothing new shows up for consumers by default — only when they attach DevTools or tail `flutter run` output.
The previous regex-driven VTT/SRT parser had nested unbounded quantifiers and could backtrack for tens of seconds on pathological inputs (e.g. 16k-cue karaoke VTTs from kaa.mx). multiDurationSearch was also O(n) per call, which then dominated the UI thread on every position event. Three changes: - Replace the VTT/SRT regex match with a hand-rolled cue-block parser that tokenizes the file into blank-line-separated blocks and only regex-matches each timing line. Linear time, no backtracking. TTML/DFXP keep the regex path. - After parsing, collapse identical-time cues to the longest text and merge contiguous identical-text cues. Karaoke files shrink from ~16k cues to a few hundred without losing visible content. - Rewrite multiDurationSearch as upper_bound on start + backward walk bounded by a precomputed prefix-max-end array (O(log n + k)). Validated on a 733KB / 16k-cue VTT: parse+sort+cleanup 67s -> 126ms, 539 cues remain, 1000 searches in <1ms. Test added uses a synthetic karaoke VTT generated in-memory.
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