ASSA: new lines keep the neighbouring style instead of the first one in the header

[niksedk]

Fixes #13677.

A line inserted into an ASSA file adopted whichever style sat first in the header, so a file whose Default style was not listed first got every new line in a foreign style. In the reported file the style order is Julia Lepetit, Jacob Andrews, Both, Default — Default carries all 2369 lines but sits last, so new lines came out as Julia Lepetit.

Two causes

The waveform and video-position inserts never assigned a style. They go straight to InsertInCorrectPosition, which has no style logic, so the line was left with an empty style — visible as a blank Style cell in the reporter's video. AdvancedSubStationAlpha.ToText then resolves an empty Extra to styles[0] on save, even when the header does contain a style named Default. SE 4 called SetStyleForNewParagraph on these paths; the SE 5 port dropped it.

The paths that did set a style threw the neighbour away. InsertService copied the neighbour's Extra and then overwrote Style with the first header style. SE 4's ASSA branch ended at newParagraph.Extra = c.Extra — the override line is new in SE 5. The TTML branch right above still inherits correctly, so ASSA was the outlier.

Changes

• SetDefaultAssaStyleForNewParagraph takes the index the line landed at and is now called from the nine insert paths that bypass IInsertService — waveform selection inserts, video-position inserts, waveform paste, split-at-position, and both speech-to-text paths (those two inherit the style of the line they replace). It no-ops when the line already has a style.
• GetStyleNameForNewParagraph picks, in order: the neighbouring line's style, the style flagged as default in the styles storage, a style named Default, and only then the first style in the header. The storage's "Set style as default" flag used to be ignored the moment a file had styles of its own — the second half of the report.
• The ASSA and SSA style storages are seeded with the built-in Default style on first start and after a settings reset, so "Styles saved" is not empty and there is something to make default. The seeded style matches libse's built-in default header, so a fresh install converts to ASSA exactly as before.
• Drive-by: InsertService clamps the insert index. InsertAfter asked for index 1 on an empty subtitle and threw; every caller happens to guard against it today, so it was latent.

Tests

tests/UI/Logic/InsertServiceAssaStyleTests.cs (new) drives the real InsertService with the reporter's style list: neighbour inheritance both directions, a blank neighbour falling through to Default, the written file's Dialogue fields, SubRip left untouched, and the clamped insert positions.

AssaStyleStorageHelperTests covers the new resolution order plus two seeding invariants: the seeded style produces the same styles as libse's built-in default header, and a save/load round-trip keeps one style — proving deserialization replaces the seeded list rather than appending, which would otherwise give every existing user a duplicate Default on each launch.

Full UI suite: 2824 passed, 0 failed.

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