feat(editor): group drag for a barline multi-selection (tempo PR 5b)

[ChrisBeWithYou]

What & why

Tempo-track PR 5b (charrette arch 1, second half). Completes the barline-selection feature #226 (5a) started: after selecting multiple barlines, you can now drag the whole group at once by grabbing any pole in it — not just delete them. This is the natural way to nudge a whole passage's grid onto the recording.

⚠ Stacked on #226 (feat/editor-tempo-multiselect). Base is that branch so the diff is 5b-only. After #226 squash-merges, rebase onto main: git rebase --onto main feat/editor-tempo-multiselect feat/editor-tempo-groupdrag (GitHub auto-retargets the base).

Behaviour

• Grab any pole in a multi-selection → the whole group slides by one offset.
• Spans between two selected poles rigid-shift (their tempo is preserved); spans at the selection's edges re-space against the fixed barline just outside — exactly like a single-pole drag generalised.
• The group stops the moment any member would collide with a fixed neighbour (± the minimum measure), so it can never reorder the grid. Song start / duration bound the ends.
• Notes ride the move (one TempoMapCmd, undoable as Move N barlines together).
• Locked barlines are excluded — they stay put and act as fixed anchors the group re-spaces around; the status says how many stayed. A lock's job is to defend a hand-verified time, so excluding it is the least-surprising rule.

How

Two pure functions in src/tempo.js:

• _tempoGroupDragClampPure(beats, selIdxs, deltaT, minGap, duration) — the whole-group clamp (tightest per-pole headroom against the nearest unselected downbeat).
• _tempoApplyGroupDragPure(beats, selIdxs, deltaT, minGap, duration) — shifts the selected downbeats, re-spaces every span's interior proportionally between its new endpoints, rigid-shifts a leading pickup / trailing tail when its bounding downbeat moved. Never mutates the input; drops locked poles from the group.

Wiring mirrors the existing single-pole drag: a mousedown on a pole that belongs to a 2+ selection starts a tempo-group drag; _tempoMapOnDragMove rebuilds from the original grid each move (no compounding); the shared _tempoMapOnDragEnd finalises it as one equal-count TempoMapCmd. The multi-selection is index-preserving, so it survives the drag. (src/mouse.js routes the new drag type.)

Verification

• npm test — 116 pass. New tests/tempo_group_drag.test.mjs (12 cases): clamp math both directions, a fixed pole between two selected ones binding both sides, rigid interior + edge re-space, pickup/tail shift, locked-pole exclusion, all-locked no-op, purity, and a full exec → undo → redo round-trip that also checks a note rides. Fails on main (the pures don't exist).
• npm run lint — 0 errors (3 pre-existing ratchet warnings).
• Live on the testbed: entered Tempo Map on AC/DC — Back In Black, Ctrl+A (105 barlines), dragged a pole → "Moved 105 barlines together — notes ride the grid.", undo enabled, no page errors.

🤖 Generated with Claude Code

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