[feat] 15-G convert to mesh (materials preserved, one undo) + multi-select menu audit

[AlexZ005]

Executes plans-core/pending/15-g-convert-to-mesh.md (roadmap #15 batch G).

Feature

Context-menu Convert to mesh for (a) a selected Group, (b) a multi-selection of 2+ meshes. The sources merge into ONE new mesh, materials are preserved as slots, the originals are deleted, and the whole thing is ONE undo entry and ONE replicated object.

How it works

• objectActions.convertToMesh(uuids) merges through mergeGeometries. Each source geometry is normalized first (non-indexed, position/normal/uv only, generated normals/uvs when a source lacks them, morph attributes dropped) and baked relative to the FIRST source's world position, so the world layout survives while local coordinates stay small.
• Materials survive. One geometry group per source material slot, deduplicated by material identity, then re-pointed onto the merged groups — mergeGeometries numbers its groups by INPUT order, so the dedup mapping has to be written back. A source that already carries a material ARRAY is split along its own geometry groups first: mergeGeometries writes exactly one group per input geometry and ignores the groups already on it, so re-merging a previously-merged mesh would otherwise silently drop every slot but the first.
• The multi-select emissive highlight is stripped from the cloned materials — same mechanism as the 15-B2 duplicate bug (a clone made from a tinted source keeps the selection blue forever).
• ONE undo entry: the deletes and the create run inside beginHistoryBatch()/endHistoryBatch('Convert to mesh'), the groupSelection template.
• Replication deviates from the plan. The plan said addImported, which ends in sendObjects(null, mesh) — but that helper announces a GROUP for the root it is handed and then walks its CHILDREN, so a bare mesh would have arrived on peers as an empty group. The merge goes out as {type:'object', element: mesh.toJSON()} (ObjectLoader on the receiver) instead, which is also exactly what the create/delete history entries replay, so a live convert and an undo/redo are byte-identical for peers.
• Guards: rigged (isSkinnedMesh) sources refuse with a toast, peer-locked objects refuse, viewers without edit rights refuse (converting deletes the sources), and fewer than 2 collected meshes refuses. A selection holding both a group and one of its descendants merges the child once, not twice.

Multi-select menu audit

Sweeping the remaining single-target entries shown during a multi-select, following the 15-B8 precedent (Edit mesh / Sculpt are hidden rather than silently acting on one object):

• Rename, Add note, Add flow to Scene graph, Ungroup — hidden while a SET is selected. Add note especially: the ViewportMenu path passes the sticky primary, not necessarily what is under the cursor, so it would anchor a note somewhere the user did not point.
• Properties used to call selectObject(uuid, true), which COLLAPSED the set to the clicked object. It now re-applies the SET, so the panel opens on what the menu header says it acts on.
• Bonus (the plan's optional S): ungroupObject is wrapped in a history batch — it used to be N+1 separate undo steps.

Verification

New suite tests/e2e/convert-to-mesh.test.cjs — 57 checks, all green, run against the lane server on 5182:

• merge basics (one object out, both originals gone, 2 geometry groups, 2-slot material array, both colors, no tint baked in, vertex counts add up, world span preserved)
• ONE undo restores both originals and removes the merge; redo re-converts and the redone mesh keeps its groups + geometry
• re-merging a 2-slot mesh with a box gives 3 groups / 3 materials (the multi-material split)
• group path: a Group of 3 becomes one mesh with 3 groups, named after the group
• guards: lone mesh, light-in-the-selection, peer-locked — each refuses AND changes nothing, plus an A/B showing the same pair merges once the lock is released (so the refusal was the lock, not some other guard)
• menu audit: the counted entry appears for a set and for a lone Group but not a lone mesh, the four single-target entries are hidden for a set and still present for a lone object, Properties keeps the set
• broadcast: exactly one object message, two delete messages, and NO group message (the sendObjects trap)
• ungroup = one undo
• two peers: B receives the merged mesh as a Mesh with both geometry groups, both source colors and the full geometry, and loses both originals

The tint guard and the multi-material split guard were proven by editing the fix out and watching them go red (3 FAILURES), then restoring.

Adjacent suites re-run green: multiselect-menu, context-menu-v2, context-menu-redesign, context-menu-overflow, undo, vr-mesh-undo, flow-node-undocked, particles, particles-impact, ui-fixes-15b.

npm run build passes; svelte-check baseline held at 419/62 (none of the touched files appear in the output).

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