feat(timeline): proportional stretch of multi-block selections

[Suoerix]

Dragging the boundary edge of a multi-block selection now performs a proportional stretch anchored at the opposite side of the selection, instead of resizing a single word:

• Right edge of the latest selected block → anchored at the selection start
• Left edge of the earliest selected block → anchored at the selection end
• Internal edges and single-block selections keep the plain resize behaviour

Syllable groups are deliberately NOT expanded for stretch (unlike nudge).

Behaviour details

• Live preview during the drag writes transiently to lines (no dirty flag, no history). On pointerup, the pre-drag snapshot is restored first, then committed via updateLinesWithHistory with propagateToSiblings: false, mirroring the nudge gesture — ensuring undo restores in a single step.
• Esc or pointercancel cancels and restores the pre-drag snapshot; sub-threshold drags commit nothing.
• Snap targets the dragged edge against unselected block boundaries; Mod/Ctrl bypass remains consistent.
• The requested factor is clamped to a feasible interval derived from non-selected neighbours, minWordDuration and the audio bounds; growth lands flush against the blocking neighbour instead of crossing it. Cross-line selections are clamped independently per line (consistent with nudge semantics).
• Background words ride along and are stamped with manual provenance; line-synced rows scale their begin/end with the same factor. Updates for main track + bg on the same line are merged into a single entry.
• Selection members can never start overlapping each other: the affine map (k > 0) is strictly increasing, so only neighbours and global bounds constrain the factor.
• Race condition protection: If external code modifies lines during drag (Ctrl+Z, import replacement, project clear), the gesture detects non-own writes via array reference validation on pointerup → abandons both restore and commit, letting external state win. This prevents swallowing undo results or creating history entries with stale snapshots as before-state.

Implementation

File	Role
stretch-targets.ts (new)	Selection → track partition (same routing as the nudge partitioner, minus group expansion) + feasible-factor bounds with generic anchor-affine constraint math newX = A + (x−A)·k (five clamp categories, NaN guard)
stretch-drag.ts (new)	planStretchDrag qualification: ≥2 blocks, dragged block must be selected and at selection extreme edge
stretch-selection.ts (new)	stretchSelections pure mapping (anchor: "start" | "end")
use-selection-stretch.ts (new)	Pointer gesture hook: full snapshot on pointerdown, real-time preview, snap integration, Escape/unmount restore, re-entry/unload protection
word-track.tsx	Hook-up point: handleResizeStart tries the stretch first (+18 net lines), falls back to plain resize
lines-slice.ts / types.ts	New setTransientLines explicit entry: dedicated for gesture preview, JSDoc declares contract "no history/dirty mutation, preserves array reference, caller responsible for restore and external write validation"

[boidushya]

Thanks for your contribution!