palette: give the spectral mid and the envelope trace their own roles

Decouples the keyboard strip's mid stop from accent/secondary, which the
deep-teal darkening had inverted. Adds overlay/trace (#816AA6), the first
value to clear 3:1 against the waveform. Replaces the frozen-premise test.

src/core/ui/CLAUDE.md

@@ -24,23 +24,34 @@ L7 sub-pass, 2026-07-27):
 - **DS-2 — palette is role-based, not hardcoded hue**, in one constants block
   (`theme`): `bg/base`, `bg/panel`, `bg/cell`, `line/hairline`, `text/primary`,
   `text/dim`, `accent/primary`, `accent/secondary`, `accent/tertiary`,
-  `accent/hot`, `warn`. Neutral ladder sits in REAPER's mid-grey theme family
-  (`bg/base` ≈ `#2b2b2b`, `bg/panel` ≈ `#333333`, `bg/cell` ≈ `#3a3a3a`,
-  `line/hairline` ≈ `#4a4a4a`, `text/primary` ≈ `#dcdcdc`, `text/dim` ≈
+  `accent/hot`, `overlay/trace`, `warn`. Neutral ladder sits in REAPER's mid-grey
+  theme family (`bg/base` ≈ `#2b2b2b`, `bg/panel` ≈ `#333333`, `bg/cell` ≈
+  `#3a3a3a`, `line/hairline` ≈ `#4a4a4a`, `text/primary` ≈ `#dcdcdc`, `text/dim` ≈
   `~#a0a0a0`+), elevation-ladder discipline (base < panel < cell by a few %,
-  micro-gradient + inner highlight/shadow carry elevation, not hard borders).
-  Three pastel accents carry categorical meaning: **primary (pastel lime) =
-  live/active/selected**, secondary (pastel teal) + tertiary (pastel purple) =
+  micro-gradient + inner highlight/shadow carry elevation, not hard borders). The
+  accents carry categorical meaning: **primary (pastel lime) =
+  live/active/selected**, secondary (deep teal) + tertiary (pastel purple) =
   supporting categorical distinctions (kinds, not intensity). `warn` (red/amber)
   is reserved **only** for byte-deleting or clip states (prune, delete).
+  **The role count is not fixed at three** — a shade that works better in one spot
+  earns its own named role (Daniel, 2026-07-31). What does not relax: draw sites
+  ask by role. A literal color outside `theme`'s constants block is the violation,
+  not the role count.
 - **WCAG-floor discipline (tight pairs to re-verify on any palette change):**
   `text/dim` on `bg/panel`/`bg/cell` is the classic mid-grey-on-mid-grey floor
-  failure — must clear AA 4.5:1 body text. The three pastels as state
-  indicators/active fills on `bg/cell` have a shrunken contrast cushion
-  (~6:1–7:1, still clears 3:1 but re-check on any hue nudge). Body text on a
-  pastel fill is a tight AA 4.5:1 pair. Take the most pastel value that still
-  clears the floor, approached from the soft side, never re-saturated toward
-  neon "to be safe."
+  failure — must clear AA 4.5:1 body text. The accents as state indicators/active
+  fills on `bg/cell` have a shrunken contrast cushion (primary ~7.6:1, tertiary
+  ~5.5:1, secondary only ~3.9:1 since it was darkened — all still clear 3:1, but
+  re-check every one on any hue nudge). Body text on an accent fill is a tight AA
+  4.5:1 pair, and for `accent/secondary` it is the *binding* limiter (~4.9:1), not
+  the indicator floor. Take the most pastel value that still clears the floor,
+  approached from the soft side, never re-saturated toward neon "to be safe."
+- **A mark drawn OVER an accent fill answers to two neighbours at once.**
+  `overlay/trace` (the envelope trace on the waveform) must clear 3:1 against both
+  `accent/primary` and `bg/base`, which sit 9.41:1 apart — so `sqrt(9.41)` ≈ 3.07:1
+  is the ceiling for *any* single value, and the role sits at that optimum with no
+  headroom either way. It is therefore confined to the waveform band: no value can
+  clear 3:1 on `bg/cell` and against the lime simultaneously.
 - **"Speed is the selling point" — a design constraint on this geometry, not
   just the draw layer.** Sub-frame hover/press/drag feedback must repaint
   immediately on the input message. **No decorative animation** — no
@@ -68,7 +79,7 @@ L7 sub-pass, 2026-07-27):
 - `tab_strip` — REAPER-free scrollable tab-strip layout + hit-test for the named-banks strip.
 - `prune_button` — pure layout/hit-test for the `bank_panel` footer Prune button.
 - `drag_out` — pure OS drag-out module: gesture-boundary decision and path-list assembly. The `InstrumentDrop` gesture signals that the shell should execute an instrument-drop rather than a file-copy drag.
-- `theme` — pure palette module: role→color mapping, REAPER-grey neutral ladder + three-accent pastel system, WCAG contrast-floor helpers.
+- `theme` — pure palette module: role→color mapping, REAPER-grey neutral ladder + the pastel accent system, the keyboard strip's spectral ramp (its own three stops, deliberately not aliases of the accents), WCAG contrast-floor helpers.
 - `component_geometry` — pure button/slider/list-row geometry + hover hit-test helpers.
 - `action_bar` — pure task-grouped action-bar layout/hit-test: clusters (Capture / Placement / Maintenance / Tagging / Switching).
 - `footer_bar` — pure footer layout/hit-test: `[Arrange|Design]` mode-toggle geometry, Tail button, and Prune placement.