Curve editor: tangent handle drag + resample-into-track (D3 follow-up)

[fernandotonon]

Overview

Follow-up to #380 / PR #381 — the curve editor's free-form tangent handle drag + resample-into-TransformKeyFrame.

D3a (#379) shipped the read-only view; D3b (#381) shipped per-channel values + the interp-mode picker. This issue tracks the remaining editing affordances:

• Tangent handle drag — mouse-drag the tangent stubs the curve editor already paints. Updates CurveEditModel.setTangents(). Auto-promotes the keyframe to Bezier mode (the model already does this).
• Keyframe value drag — Y-axis drag on a keyframe square edits its channel value via setKeyframeValue; X-axis drag retimes via the existing moveKeyframe. Shift-constrains to one axis.
• Resampler — when a tangent / value / mode changes, walk the affected curve segment in N samples/sec and write the result back into the underlying TransformKeyFrames so Ogre playback matches the curve. N = 30 default, bumped to 60 around high-curvature regions, capped at 200 keyframes/segment.
• Resample undo — single ResampleCurveCommand per gesture so Ctrl+Z reverts the whole edit.

Why this is a separate issue

The curve editor is a Canvas2D paint, so handle MouseAreas have to be placed dynamically at the painted positions. Doing this correctly under arbitrary curve shapes (and pan/zoom transforms) is a non-trivial QML refactor — likely worth its own PR rather than expanding D3b.

Acceptance criteria

• Drag a tangent handle → curve reshapes live; release commits one undoable resample
• Y-drag a keyframe square → channel value updates live; commit on release
• X-drag a keyframe square → time updates via moveKeyframe (already undoable)
• After editing, viewport playback follows the new curve shape
• Ctrl+Z undoes a curve edit in one step

Plan (#260)

Closes Phase 5's animation work once it lands.

[fernandotonon]

Closing — all acceptance criteria delivered across PRs #392 (live drag UX, mode picker, Shift axis-lock) and #395 (resample-into-track via Bake button + CLI/MCP, single-step undo via CurveEditModelChangeCommand + ResampleCurveCommand macro, viewport playback follows shape after Bake). Released in 2.35.0.