Make the contraption builder palette professional: collapsible sections + polish

[SethMorrowSoftware]

What & why

The left palette (sidebar) of the Contraption Builder worked, but read as unpolished: a single long scrolling column of 26 buttons whose labels didn't line up (the glyph was baked into each button label, and the Unicode glyphs have different widths), behind a dated native scrollbar, with hover driven by a fragile hardcoded-colour string match and ~66 chrome colours hardcoded inline.

This PR makes it look intentional and easier to scan, without changing any tool behaviour. All work is in one file: examples/box2dxt-contraption-builder.livecodescript.

Changes

Collapsible (accordion) sections

• Each section header (SHAPES, TERRAIN, SPECIAL, VEHICLES, JOINTS) is now a clickable button with a chevron (▾ expanded / ▸ collapsed).
• New layoutPalette repositions/shows/hides rows from gPalOpen state; hidden controls drop out of the group's scroll extent, so the scrollbar only appears when the visible content actually overflows.
• Routed through the existing mouseUp → handleUiClick dispatch (new ui_hdr_ branch + togglePaletteSection) — no new card handler. mouseDown already ignores ui_ targets, so headers never disturb physics. Default is all sections expanded, preserving today's first-run discoverability.

Visual polish

• Stopped baking the glyph into the button label. Each glyph now lives in a fixed-width gutter field (ui_g_<id>), centered, so every label lines up regardless of glyph width.
• Roomier rows/headers (button height 24→28, gap 3→4, header 18→22).

Robust hover + selection (DRY)

• New setBtnIdle / setBtnActive helpers are the single source of button styling, used by highlightGroup, makeAction, makePropBtn, setToggle, highlightActions, and the inspector-tab highlighter.
• hoverTool now derives idle/active state from the current selection (gTool/gJointTool), not from reading a pixel colour — it can't mis-read an accent or fight the selected/collapsed styling.

Easy wins folded in

• Theme constants: the repeated chrome colours are unified into one kCol* block (kColBtnIdle, kColAccentGreen, kColAccentAmber, kColDanger, …). Scene-part fill colours (red game pieces) are intentionally left as literals.
• Magic numbers extracted: kDefaultMotorSpeed (was 120) and kDefaultGravity (was -10).

A note on verification

This UI is LiveCode/xTalk script that runs only inside the LiveCode/OpenXTalk IDE — there's no headless way to run it in CI or a container, so I verified by careful static review (structure, handler balance, grouping/clearUI/raiseUI conventions, coordinate math). All edits are script-and-control only, with no new external assets, so the standalone build is unaffected.

Manual verification checklist (run in the IDE)

1. Open the card → 5 headers each with a ▾ chevron, rule, and tool rows. All labels left-align (glyphs centered in the gutter).
2. Polish: taller rows, comfortable gaps, bold light-grey headers, no clipping at the scrollbar edge.
3. Collapse/expand: click SHAPES → chevron ▸, rows hide, sections below slide up; click again → restores.
4. Multi-collapse: collapse several → content compacts and the scrollbar disappears when it all fits, returns when overflowing.
5. Scroll (all open) → native scrollbar moves only palette content; collapsed controls never flash in.
6. Select: Box → green, HUD "Tool: …"; Delete → red; a joint → amber. Selection survives collapsing/expanding other sections.
7. Hover: idle tool lightens; the selected tool does not; leaving restores correct idle/accent (no stuck colour).
8. Run / Build: Run starts the simulation and parts fall; the palette still collapses and selecting a tool drops back to Build; Build/Reset returns to edit.
9. Rebuild integrity: leave & return to the card → palette returns all-expanded; no stray ui_g_* orphans or duplicate ui_palettescroll.
10. Other top-bar actions (Clear/Save/Load/Recipes/Help) still work.

https://claude.ai/code/session_01XpBcQg2DbncrBcFZLHqhMj

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