Perf: single-raise timing notifications + cheaper ASSA event-line parsing

[niksedk]

Two hot-path fixes from the second performance review round.

Subtitle line timing notifications (grid / waveform drags)

• TextBackgroundBrush was raised from OnEndTimeChanged, OnDurationChanged, and UpdateDuration, but its getter depends only on Text (line length / pixel width / line count) — and it strips HTML and shapes text with HarfBuzz per evaluation. Waveform drags assign StartTime/EndTime per pointer move (60–120 Hz), so realized rows re-shaped their text up to ~3× per mouse event for no reason.
• OnEndTimeChanged first called UpdateDuration (which raises CPS/WPM/brushes) and then raised the same set again — every end-time assignment notified twice.
• OnGapChanged raised DurationBackgroundBrush, which doesn't read Gap.

UpdateDuration is now the single raise site for timing-derived properties (all timing mutations — OnStartTimeChanged, OnEndTimeChanged, SetTimes, SetStartTimeOnly — funnel through it). It also gains CpsBackgroundBrush/WpmBackgroundBrush, fixing a latent staleness bug: dragging the start edge changed CPS but never repainted the CPS/WPM cell backgrounds.

TextBackgroundBrush keeps its two correct raise sites: OnTextChanged and the settings-dependent refresh.

ASSA LoadSubtitle

• Every event line was copied with ToLowerInvariant() just for a handful of prefix checks (format:, dialogue:, comment:) — for a 100k-line .ass that's 100k full-line lowercase copies, doubled by the IsMine pre-parse. Now uses StartsWith(..., OrdinalIgnoreCase); only the short format field list is lowercased.
• The text-field rejoin (text += "," + part per comma in the dialogue text) is now a pooled StringBuilder.
• Reuses the already-computed trimmedLine instead of three extra TrimStart() calls per line.

All 997 tests pass (libse 485, UI 512).

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