Add Chroma: shared color roles, gradients and capability fallback

[raiseCatError]

Implements #171.

Stacked on #219 (feature/170-accessibility-baseline), on #218, #217 and #203. Merge order: #203, #217, #218, #219, then this PR.

What changed

• src/chroma/chroma.ts: color references in three separate categories: status (success/failure/working), theme (primary, accent, selection and so on) and identity (a thing's own color, such as a language). statusMeaning() returns a value only for status colors, so an identity color can never read as success or failure. Also solid colors, linear and multi-stop gradients, easing curves (linear, ease-in, ease-out, ease-in-out), luminance and gradientText.
• src/chroma/escape.ts: one place that turns a color into an escape sequence for truecolor, 256-color or no color. ui/palette foreground()/background() now go through it, so every existing panel follows the capability level.
• NMSH_COLOR=256 selects the 256-color mapping; automatic detection is not added.
• Consumers: the shimmer uses Chroma's mixing, and Settings status tones use the status and theme roles.
• BRAND_LAVENDER (#A67CF3) is exported as a default, and nothing depends on it.

Boundaries

• Chroma chooses colors; it does not place cells. It is not a layout engine and does not duplicate the Chroma: gradients, animated color treatments, and transient visual effects #78 research.
• The Standardized language identity colors from GitHub Linguist #176 Linguist dataset is not included. Only the identity category boundary exists.
• Raw PTY output and /copy are untouched.

Verification

tests/chroma.test.ts; full suite 642/642, typecheck, build and git diff --check clean.