feat(generate): multi-level mesh undo with a bounded per-model stack - #404

Closes #304

Finding

Edits never overwrite their input: every op (/optimize/op/{id}, /arrange, /transform, decimate, smooth) writes a new GLB next to it, and nothing deletes them. A stack of URLs is therefore enough to restore earlier meshes, with no snapshot copies and no extra disk use.

The store already had a URL history with redo. What was broken:

1. Unbounded: the history had no size limit.
2. Crossed models: setCurrentJob never reset it. After an import, a library open, Add to Scene or a workflow run, undo jumped back to the previous model.
3. First edit not undoable: callers set outputUrl before pushing, so a mesh that reached the viewer unrecorded (workflow output, Add to Scene) couldn't be undone back to.

Changes

• New pure module meshHistory.ts: a stack capped at 20 undo steps, with record / record-edit / step back / step forward and undo/redo depth.
• Single edit entry point applyMeshEdit(url): it records the edit from the mesh on screen, seeding that mesh first if it isn't current. Smooth, decimate and agent mesh_update results all go through it.
• Loads: pushMeshUrl now only records a freshly loaded mesh.
• Reset per model: setCurrentJob resets the stack to the new job's mesh, even for the same id, since Add to Scene reuses workflow-output.
• UI: Undo/Redo are disabled when there's nothing to step to, the tooltips show the step count, and Undo has a depth badge.

Merge note

Conflicts with #403 on the test:node line of package.json only (both add a test file). Whichever merges second needs a one-line rebase; I'll handle it.

Not done

• The Undo on a chat actions card still steps back once, not "the whole turn".
• An agent edit finishing after a model switch still lands on the new model. This is pre-existing; a job-id guard would fix it.
• History is in memory only, as before.

Test plan

• meshHistory.test.ts (new): 12/12, including "3 edits then 3 undos restore the initial mesh"
• pre-push hook (npm test, full suite) passed
• eslint on the changed files: clean
• Manual: smooth → decimate → agent edit, then 3× Undo / Redo; switch model and check Undo is disabled