Add Unbreak / Auto-break / Split-rebalance / Evenly distribute to Selected Lines menu

[niksedk]

Summary

Four line-shaping operations were missing from SubtitleGrid → Selected Lines in the context menu. This PR wires the existing Unbreak and AutoBreak commands into the submenu, adds a "Split/rebalance long lines…" entry that opens the existing Tools dialog scoped to the selected lines, and ports SE4's "Evenly distribute lines" as a new command.

Changes

Unbreak / Auto-break (one-line additions)

The view-model methods already exist (MainViewModel.cs:10704 / 10721) and iterate SubtitleGrid.SelectedItems. Just added menu entries bound to UnbreakCommand / AutoBreakCommand. Reuses Se.Language.General.Unbreak and Se.Language.General.AutoBreak (which already exist in every translation).

Split/rebalance long lines (new — opens existing dialog scoped to selection)

ShowToolsSplitBreakLongLinesSelectedLinesCommand:

1. Snapshots the lowest selected index.
2. Opens the existing SplitBreakLongLinesWindow with only the selected SubtitleLineViewModels.
3. On OK, removes the selected lines from Subtitles and splices the dialog's AllSubtitlesFixed result in at the lowest selected index.

Non-contiguous selections are compacted together at that index. The existing full-subtitle entry under Tools menu is unchanged. Reuses Se.Language.Main.Menu.SplitBreakLongLines.

Evenly distribute lines (by CPS) — new, ported from SE4

EvenlyDistributeSelectedLinesCommand ports Forms/Main.cs:22773-22822 from se4-legacy. Algorithm:

• Take the time span from the first selected paragraph's start to the last selected paragraph's end.
• Subtract (SelectedCount - 1) × MinimumBetweenLines.
• Distribute the remaining duration proportionally to each paragraph's CountCharacters(true) (CPS-equalizing redistribution).
• Pack back-to-back with the configured minimum gap.

Only visible when more than one line is selected (HasMultipleLinesSelected binding). New language strings:

• Main.Menu.EvenlyDistributeLines = "Evenly distribute lines (by CPS)"
• Main.NumberOfLinesEvenlyDistributedX = "Evenly distributed {0} lines" (status message)

Menu structure

The four new entries are wrapped in Separators to keep line-shaping operations together and away from the unrelated entries above and below:

Selected lines >
  Speech to Text
  Text to Speech
  Auto Translate
  Change Casing
  Set Layer
  Fix common errors
  Multiple replace
  ─────────
  Unbreak text                ← new
  Auto-break text             ← new
  Split/rebalance long lines… ← new (opens dialog scoped to selection)
  Evenly distribute lines (by CPS) ← new
  ─────────
  Fill with clipboard
  Surround with 1/2/3
  Cut video
  Statistics

Test plan

• Select several lines, right-click → Selected Lines → Unbreak text. Confirm only the selected lines have line breaks removed.
• Same with Auto-break text.
• Select a few lines with one long line. Open Split/rebalance long lines… from the context menu. Confirm the dialog opens populated with only the selected lines; on OK the result replaces just those lines.
• Non-contiguous selection + Split/rebalance: the result still gets spliced in at the lowest selected index (the other selected items elsewhere in the grid are pulled together at that position).
• Select two adjacent lines (one short, one long). Evenly distribute lines (by CPS). The long line should get more time than the short one, both packed back-to-back with the configured min gap.
• Single-line selection: Evenly distribute is hidden.
• Confirm the existing Tools → Split/rebalance long lines… still operates on the full subtitle.
• Build clean; libse tests pass (367/367); UI tests pass (306/306).

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