Report ActionClip conflicts instead of panicking and fix field span offset

[Sheerwin02]

Sequence::push and Sequence::extend guarded against overlapping
clips with a debug_assert!. That meant an authoring mistake took the
whole app down in a debug build, and in a release build the clips were
accepted silently — the animation came out wrong with nothing to point
at why.

Conflicts are now reported through tracing::error! and the clips are
kept. Reporting a conflict is not the same as resolving one, so the
animation is still wrong; you just find out immediately, in either
profile, instead of guessing.

Two things get reported, because baking walks a lane in list order
carrying one value forward (start = end in pipeline::bake) while
playback runs in time order:

• A clip listed after one that starts later. No overlap needed —
  the two orders simply disagree, so the later-listed clip opens on a
  value that has not happened yet.
• A clip overlapping any earlier clip in the lane. Every offender
  is named, not just the nearest one, so a clip buried under a long
  one is reported even when the clips either side of it are clear.

Adds tracing as an optional dependency, enabled by default. With the
feature off the checks compile out entirely.

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Also fixes field_offset in TrackFragment::compile assigning rather
than accumulating, which made every field lookup past the second one
point at the wrong lanes — field_offset = field_len overwrote the
running position with the size of the previous run. The first boundary
was accidentally correct (the offset is still zero there), so it only
shows up with three or more distinct fields.

This is latent: nothing reads field_lookups yet, per the TODO on
Track. The added test fails without the fix and asserts the spans
partition the lanes , every lane reachable through exactly one field,
none skipped, none repeated.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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Plan: Pro Plus

Run ID: 5858425b-f7a6-4a5e-942c-ee08d84b0154

📥 Commits

Reviewing files that changed from the base of the PR and between dc132ce and 393a1fe.

📒 Files selected for processing (1)

• crates/motiongfx/Cargo.toml

🚧 Files skipped from review as they are similar to previous changes (1)

• crates/motiongfx/Cargo.toml

Included review availability: Your plan includes up to 2 reviews per rolling hour; 0 remain after this review.

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added diagnostic reporting for overlapping or out-of-order clips during sequence updates, rather than halting the operation.

• Bug Fixes

  • Corrected compiled track field offsets so lookups consistently cover every sequence lane.
  • Improved conflict detection when adding clips individually or in batches.

Walkthrough

The PR adds tracing-based conflict reporting for sequence insertion and extension. It also fixes cumulative field offsets in track compilation and adds regression coverage for field-to-lane lookups.

Changes

Sequence and track corrections

Layer / File(s)	Summary
Sequence conflict reporting crates/motiongfx/Cargo.toml, crates/motiongfx/src/sequence.rs	The crate enables tracing by default. Sequence::push and Extend::extend report out-of-order and overlapping clips instead of rejecting them with debug assertions.
Track field offset correction crates/motiongfx/src/track.rs	Track compilation now advances field offsets cumulatively. Regression coverage verifies field lookups, key ownership, and complete lane coverage.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 393a1

The PR replaces debug-only panics with error reporting and corrects field span offsets while preserving existing clips; no actionable merge-blocking risk remains beyond normal checks and review.

Possibly related PRs

• voxell-tech/motiongfx#124: Both PRs configure the optional tracing dependency in Cargo.toml.
• voxell-tech/motiongfx#125: Both PRs modify track.rs and compiled track lane handling.
• voxell-tech/motiongfx#134: Both PRs modify sequence conflict handling and track field-offset accumulation.

Suggested reviewers: jaghov

Poem

A rabbit traces clips in flight,
And marks each conflict clear and bright.
Field offsets move in steady rows,
Each lane is found where it should go.
The compiled track stays on track.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes both main changes: reporting ActionClip conflicts and fixing field span offsets.
Description check	✅ Passed	The description accurately explains the conflict reporting behavior, feature configuration, and field span offset fix.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[nixonyh]

A small nit, good to merge once addressed.