UI system: ambient instrument core + BASE status always-on (fixes per-theme status bug)

[tamirelazar]

Part of the UI-system unification. Introduce the ambient instrument — one bottom-docked surface with a BASE/TUNE/MSG state machine — and land the BASE (status) state on it.

What to build

• An ambient surface module with a state machine and resolution priority: error-MSG > MSG > TUNE > BASE; expired non-sticky states fall back. The surface always reserves the taller (tuner) height so later states do not cause layout jump.
• BASE = the status line, reborn on the shared RowBuf + L1 tokens. This fixes the standing bug where the status line hardcodes Gruvbox colors and is therefore visually wrong on the other four themes.
• BASE is the resting state and renders at rest. This is a behavior change, not a port: today the status line only appears when an overlay is open or paused. The at-rest render gate must be removed so BASE shows during normal running (gated only by the user status-bar/chrome preference).

State machine shape (from the design):

• BASE
• TUNE { param, until }
• MSG { level, text, sticky, until }
  resolve(states, now) picks by the priority above after dropping expired non-sticky entries.

Acceptance criteria

• State machine + resolve priority unit-tested (error outranks all; expired TUNE -> BASE; live MSG outranks TUNE)
• BASE renders from tokens; a non-Gruvbox theme golden differs from Gruvbox in status colors (proves the bug is fixed)
• BASE renders at rest (no overlay, not paused); surface reserves tuner height
• visual-regression + cargo test --lib + clippy pass

Blocked by

• UI system: L2 shared primitives (merged RowBuf + state mapping + widget kit) #62
• UI system: rewire Controls (console/tuner) onto L2 primitives #63
• UI system: motion substrate (phase clock + motion helpers + composite dim, drop tachyonfx) #61

[tamirelazar]

Delivered in PR #71 (UI-system unification, merged to dev). This slice landed as part of the bundled SDD implementation; verified present in dev.