feat(tui): greet launch with the mark, once, drawn in braille

[benjipeng]

What

Stage 10: launch greets a new conversation with the mark, once.

• The greeting. On a launch onto an empty conversation the mark plays at the conversation's centre with the name beneath and leaves after about 2.5 s: frame fades in (0.25 s), the chosen cycle runs in 2 s (a dot grows into a circle, becomes a rounded square and a square, turns into a diamond while the frame turns with it, shrinks away) with one 0.8 s sheen over the frame, then frame and name fade out (0.3 s). Input never waits; the first message ends it for good; /new, /resume and reopened conversations are not greeted. The empty primary conversation it leaves is blank (the inspector keeps its placeholder).
• The drawing. Braille, after Grok's welcome logo (read from its source): two by four dots a cell, so the mark is a small bitmap squared against the terminal's reported cell (crossterm::terminal::window_size), a common monospace cell standing in. Colours come from the palette's slots (blue frame lit toward text, purple centre, fading from the ground), so a theme reaches them.
• The clock. The greeting counts the one motion clock's phases from the instant launch greeted, plays its 38 once, and stops waking the clock (MOT-1/MOT-2/MOT-3).

The user chose the drawing, motion, greeting and pace in kitty from the spike's previews (kept as mockups/mark-braille*.py), rejecting the box-drawn frame around a one-cell glyph (too coarse), a mark that stayed until the first message (too insistent), 3.7 s (too slow) and a placeholder sentence afterwards; the Rust output was compared with the chosen preview dot for dot. ui-ux, the motion spec and evidence, the spike and the phase say so; the plan is consumed. Frames: crates/plexmaton-tui/frames/greeting/ at 120/88/60.

scripts/smoke-tui.py's "a status-line press repaints nothing" check now waits for the greeting to leave, since the greeting repaints while it plays.

Verified

Local: cargo test --workspace --no-fail-fast 1639 passed; all eight terminal smokes; workspace clippy -D warnings; fmt, file length, citations, frames, typos, script unit tests. The user watched the greeting in the real binary. CI not yet observed.