Panel sections that fold and fill cleanly, a shaded canvas well, and light by default

[thethinkmachine]

What changed

Left panel: collapsing and expanding sections

• States filter no longer stretches. With Transitions folded (or States torn off into a tall window), the filter box grew to ~300px and stranded the rows below it. .search-field had flex: 1, which grows on the column axis.
• The fill list sizes to its rows. The last open list took all of the panel's spare height whatever it held, so five states stood over ~500px of empty panel with the folded Transitions header pinned to the bottom. It now grows past the 168px cap only as far as its rows reach, and shrinks to the room when it is longer. panel-list.js publishes the list's floor and the section's floor (--lw-floor, --dock-fill-min), because Chrome takes a flex or grid item's automatic minimum from its whole content (12,000px for a 200-state list).
• Grip and pop-out no longer stay lit after a click. They used :focus-within on a tabindex="0" header; now they show on hover and keyboard focus only.
• The grip sits in the header's padding, with room before the label. It used to reserve 18px of the row, which pushed the Transitions collapse arrow past the edge at the 220px minimum. Title, status chip and badge now give way by priority: chips truncate (full text on the tooltip) before the title clips.
• δ table header framed where its rows are. It was a gutter wider than the rows, and the whole table sat a gutter narrower than the fields even with nothing to scroll. The header row draws the frame and is padded by the measured scrollbar width, kept current by a ResizeObserver.
• Section rules reach the panel edge. The panel bodies dropped scrollbar-gutter: stable, which reserved an empty 10px strip.
• A folded multi-alphabet section shows Σ2 · Γ3 · Δ2 instead of a bare "ALPHABETS".
• Transition rows keep state names whole (carry 0, not ca…), truncating the label first.

Both panels: consistency

I measured every docked section header across both panels and all tabs. Height, padding, title type and position, arrow, grip and borders now match. Two real drifts are fixed:

• Section body padding: left was 3px 14px 12px, right 4px 14px 14px. Both use one token now.
• The Language badge is 16px tall like the count chips (it was 17px).

The filter text size now matches the other entry fields.

Canvas

• A light inset shadow where the header, both panels and the sill meet the canvas. It is an overlay above the diagram and below every control (z 40 vs 49+). It takes no pointer, and its colour is the theme-tuned --tab-overflow-shadow.
• The status pill gets the shadow the rest of the floating canvas chrome already had.

Light is the default theme

• The base :root block is now the light palette plus the shared tokens, and Dark is an override block like every other theme.
• All 72 colour variables are restated by every theme, so none inherits a light value by omission.
• First paint follows: the theme-color meta and the Electron window background are light.
• A stored theme choice, dark included, is kept.

Tests

• npm test: 2,790 pass.
• New tests/panel-sections-css.test.js (9 stylesheet-level pins, since the DOM stub computes no styles). All of them fail against the old CSS.
• The folded-alphabet summary is covered in tests/workspace-sections.test.js.
• tests/themes.test.js now takes the dark block as its reference for "every override restates the same variables".
• Checked in headless Chromium:
  • DFA, PDT, MTM, PFA, DPA, PDA, and 150- and 200-state machines
  • panel widths 220px and 256px
  • a short 520px window
  • Blocks as the fill
  • a real scrollbar on the δ table
  • dark, light and sepia themes
  • a fresh profile, and a stored dark choice
• npm run build succeeds.

Not in this PR

deriveRegex throws RangeError: Invalid string length on a 60-state cross-linked DFA. The regex expands past V8's string limit well under REGEX_DERIVE_MAX_STATES = 120. Found while testing; left for its own change.

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