Slip Edit in UI

[s00500]

• Slip Edit: First working version
• Slip Edit: Preview start and end frame

(Closes #300)

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Note

Medium Risk
Touches core trim/slip timeline logic and clip mutation paths; behavior is well-tested but mistakes could corrupt clip trims or A/V sync when linked.

Overview
Adds slip editing so users can shift which source frames play inside a clip without moving it on the timeline. The new Trim (T) tool mode keeps edge trims on the handles and starts a slip drag from the clip body; Escape cancels an in-progress slip without committing.

commitSlip and related helpers move trimStart/trimEnd in opposite directions with clamps for speed, linked partners, nested sequences (effectiveTrimEnd), and exclusions for multicam, image, and text clips. The timeline shows a live source-range overlay while dragging, and the viewer shows an FCP-style two-up (SlipTwoUpView) of start/end source frames during the drag.

Toolbar and T shortcut select the tool; unit tests cover slip behavior, propagation, and undo.

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[s00500]

Usage: Press T or select the double arrow next to the cut tool to enter slip mode :-)

[htin1]

tested it out. looks pretty good! i think something I'd want in terms of UX(similar to Davinci):

when dragging, I wasn't sure where I can go. In Davinci, it shows you a "ghost" frame that tells you the position of the current trim part. It also shows you which direction you can go with the green and red. ref: https://www.youtube.com/watch?v=WLOaont2TCs

I can also add a follow up PR, if that's too much for this PR (I'll let you decide if you want to include it)

[htin1]

can we double check if trackIndex is used at all?

from ai review:
trackIndex is never read: mouse-drag uses grabFrame and the delta caps, mouse-up and the draw
path use clipId/deltaFrames/propagateToLinked. Can be dropped from the struct and the init call
site.

[s00500]

will check

[s00500]

So... checking on the lsp, references that us it are in TimelineInputController.swift ;-)

So removing it gives a compile issues...

[s00500]

maybe I was a bit fast on that call, checked again and removed it :-)

[s00500]

I will get back to this in the next 2 days :-)

[htin1]

@s00500 it should be good once ci is fixed (just need to rebase i think)

[s00500]

Rebased on main :-)

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[htin1]

lgtm, thank you!

[s00500]

Awesome!!! Thanks!