UI system: motion substrate (phase clock + motion helpers + composite dim, drop tachyonfx)

[tamirelazar]

Part of the UI-system unification. Hand-rolled motion substrate (verdict: hand-roll, drop the tachyonfx dependency — it beat the library on every axis and the library was unused). Independent of the token/widget foundation; can run in parallel.

What to build

A small, allocation-light animation layer that every overlay fade and the identity "breath" ride on.

• A persistent phase clock on the runtime state, advanced each frame by dt, that outlives overlay open/close (so fade-OUT can play).
• A motion helper module: lerp_rgb, ease curves (ease_out_cubic, ease_in_out), a bounded breath oscillator, dim_overlay, crossfade, clip_rows.
• Extend the composite stage so a rich overlay can be drawn with a dim: f32 (lerps set fg/bg toward the background). Existing draw path becomes a dim = 1.0 wrapper so current renders are byte-identical.
• Remove tachyonfx from the manifest (terminal feature + deps). Note: there is no spike code to delete on this base — motion is greenfield here.

Acceptance criteria

• Phase clock advances by dt and wraps safely (unit test)
• Motion helpers unit-tested (lerp endpoints, ease clamping, breath bounded)
• dim = 1.0 composite path leaves all existing visual-regression goldens unchanged
• tachyonfx removed; cargo build --features terminal + cargo test + clippy pass

Blocked by

None - can start immediately

[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.