Add surface primitives: frames, fills and layout

[raiseCatError]

Implements #165.

Stacked on #220 (feature/171-chroma), on #219, #218, #217 and #203. Merge order: #203, #217, #218, #219, #220, then this PR.

What changed

• src/ui/surface.ts: renderSurface(rows, columns, spec) places rows in a surface. The spec has:
  • frame: none, top line, square, rounded, double or heavy;
  • frame color and fill, each either a Chroma color or a left-to-right gradient;
  • padding, inset, width (fill or content) and alignment.
• Frame and fill are independent. No frame with a fill, a frame with no fill, both, and a gradient fill are all covered by tests.
• Surfaces only hold Chroma color references. Color resolution and capability fallback (truecolor, 256, none) stay in Chroma.
• Fills stay behind styled content: the background is re-applied per glyph, so resets inside a row do not punch holes.
• Safe glyph mode draws ASCII frames.
• Narrow widths degrade padding first, then boxed frames fall back to a top line. Rows never exceed columns. With no color, geometry is kept and fills and frame colors disappear.

Consumer

• framePanel in PanelShell is now a top-line surface using the theme separator color, so every existing panel (Settings, palette, layout, syntax, transcript, providers) goes through it. Output is unchanged; tests assert that.
• The other frame and fill styles are available but no panel uses them yet. That is intentional: this PR does not redesign panels.

Verification

tests/surface.test.ts; full suite 649/649, typecheck, build and git diff --check clean.