feat(studio): add ripple, roll, slip and slide trim tools

[miguel-heygen]

What

Four dedicated trim tools for the Studio timeline, alongside Select and Razor:

Tool	Key	What it does to the track
Ripple trim	T	Trim a clip edge; every later clip on the track follows, so no gap or overlap appears.
Roll edit	⇧T	Move the cut between two butted clips; one grows by exactly what the other gives up, nothing downstream moves.
Slip	Y	Change which part of the source plays. Position and duration are untouched.
Slide	⇧Y	Move a clip along the track; its neighbours absorb the move.

Closes #3285 (except the optional FCP-style precision view — see "Not covered").

Why

Timeline trim only ever changed the grabbed clip's own in/out. On a track with
more than one clip that means every trim opens a gap or an overlap that has to
be closed by hand, and there is no single action for "trim this and shift the
rest to match". Roll, slip and slide had no equivalent at all.

Demo

Recorded against a real Studio on a track of A 0-8, B 8-12, C 12-16 (plus a
sub-composition on a second lane). Every clip below is the actual pointer
gesture, not a mock-up.

Ripple — drag A's out point right; B and C follow, and the second lane is
untouched.

Roll — drag the A|B cut; A grows by exactly what B gives up and C never
moves.

Slide — drag B along the track; A stretches and C shortens to absorb it, so
the track keeps its total length.

Slip has no motion to record — that is the whole point of it. On the
sub-composition clip it moved the in point 3s → 4.5s while start and
duration stayed at 4 + 6. On a clip with no source at all it refuses up
front and says why, instead of starting a gesture that does nothing:

How

• timelineTrimOps.ts is pure math with no store or DOM in it:
  resolveTrimPlan decides which clips an operation touches (and returns null
  when the gesture is impossible), resolveTrimDeltaBounds says how far it may
  travel, applyTrimDelta produces the per-clip patches. Semantics follow FCP /
  Premiere: a head ripple keeps the clip's start and pulls the lane in
  behind it, which is what makes it a ripple rather than a plain trim.
• timelineTrimSession.ts is the gesture layer. A trim session is a
  TimelineGroupResizeSession, so the whole existing pipeline downstream —
  projection rendering, escape-cancel, and the atomic single-undo commit through
  commitTimelineGroupResize — is reused unchanged. Only the preview math
  differs. No new persistence plumbing.
• All four are lane-scoped (matching the existing gap tooling), clamped by
  available source media and the minimum clip duration, and snap the edge that
  actually moves, with every clip that rides along excluded from the snap grid.
• A tool that cannot act on a clip refuses at pointerdown and says why,
  reusing the existing blocked-edit report — which was already being handed an
  intent it threw away.
• Tool metadata (label, shortcut, tooltip) has one owner, read by the toolbar,
  the keyboard handler and the shortcuts panel.

Two adjacent cleanups came along because the repo gates flagged them once these
files were in scope: the duplicated remove-element POST behind the two delete
paths moved into one helper, and the timeline shell's inline style moved into
the theme module (both files were at the 600-line cap).

Test plan

• Unit tests added/updated — 43 new tests: the trim math (bounds, clamps,
  playback-rate scaling, adjacency invariants for every op), the session
  layer (lane scoping, capability refusal, snapping), and six end-to-end
  gesture tests through the drag hook asserting exactly what gets persisted.

• Manual testing performed — drove a real Chrome against Studio and read
  back the project file after each gesture rather than trusting the UI.
  Each gesture dragged +2s:

  Gesture	Result on disk
  Ripple A's out	A 0+10 B 10+4 C 14+4 — second lane untouched
  Ripple B's in	A 0+8 B 8+2 C 10+4 — B's start pinned, lane closed behind
  Roll A|B	A 0+10 B 10+2 C 12+4 — C never moved
  Slide B	A 0+10 B 10+4 C 14+2 — track length unchanged
  Slip the sub-comp clip	4+6 unchanged, in point 3 → 4.5
  Slip a source-less clip	refused, nothing written, toast shown
  Plain trim / plain drag (Select)	unchanged from before this branch

  One ⌘Z after a ripple restored the whole lane, confirming the batch is a
  single undo step. All four shortcuts (T, ⇧T, Y, ⇧Y), V back to
  Select, and the toolbar buttons were driven through the real UI.

• Full packages/studio suite green (4308 tests), typecheck, oxlint, oxfmt.

Not covered

• The FCP-style two-up precision view from the issue (marked optional there)
  is not in this PR; it is a separate component and follows on its own branch.
• Cross-track (sync-locked) ripple. Every operation stays on the grabbed
  clip's own lane, matching the existing "close gap" tooling.
• Pre-existing, left alone deliberately: a plain Select-tool trim writes
  data-playback-start onto clips that have no source at all (a div gets
  in=0 on an out-trim, in=2 on a head-trim). The trim tools here do not do
  that — they only write an in point on a clip that has one — but fixing the
  shared plain-resize path changes what every head-trim writes and wants its own
  change with its own regression tests. The root is that the runtime clip
  manifest reports playbackStart: 0 for every element, so nothing downstream
  can tell "no in point" from "in point at zero". Happy to pick it up next.

Size

~1.6k changed lines against the 1k guideline, about 900 of which are the new
pure-math and gesture modules and ~450 are tests. The four operations share one
plan/bounds/apply module and one session type, so splitting them into two PRs
would cut that module in half and leave the first half untestable on its own.
Flagging rather than hiding it.

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