feat(subtitles): add no-jump subtitle-delay shift commands

backlog/tasks/task-83 - Jellyfin-subtitle-delay-shift-to-adjacent-cue-without-seek-jumps.md

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+---
+id: TASK-83
+title: 'Jellyfin subtitle delay: shift to adjacent cue without seek jumps'
+status: Done
+assignee: []
+created_date: '2026-03-02 00:06'
+updated_date: '2026-03-02 00:06'
+labels: []
+dependencies: []
+priority: high
+ordinal: 9003
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+
+Add keybinding-friendly special commands that shift `sub-delay` to align current subtitle start with next/previous cue start, without `sub-seek` probing (avoid playback jump).
+
+Scope:
+- add special commands for next/previous line alignment;
+- compute delta from active subtitle cue timeline (external subtitle file/URL, including Jellyfin-delivered URLs);
+- apply `add sub-delay <delta>` and show OSD value;
+- keep existing proxy OSD behavior for direct `sub-delay` keybinding commands.
+
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+
+- [x] #1 New special commands exist for subtitle-delay shift to next/previous cue boundary.
+- [x] #2 Shift logic parses active external subtitle source timings (SRT/VTT/ASS) and computes delta from current `sub-start`.
+- [x] #3 Runtime applies delay shift without `sub-seek` and shows OSD feedback.
+- [x] #4 Direct `sub-delay` proxy commands also show OSD current value.
+- [x] #5 Tests added for cue parsing/shift behavior and IPC dispatch wiring.
+
+<!-- AC:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+
+Implemented no-jump subtitle-delay alignment commands:
+- added `__sub-delay-next-line` and `__sub-delay-prev-line` special commands;
+- added `createShiftSubtitleDelayToAdjacentCueHandler` to parse cue start times from active external subtitle source and apply `add sub-delay` delta from current `sub-start`;
+- wired command handling through IPC runtime deps into main runtime;
+- retained/extended OSD proxy feedback for `sub-delay` keybindings;
+- updated configuration docs and added regression tests for subtitle-delay shift and IPC command routing.
+
+<!-- SECTION:FINAL_SUMMARY:END -->