fix: draw a frame only when there is something new to show

[vyncint]

Closes #102.

The bug

app.redraw gated only the full clear, not the draw itself, so the loop bracketed and drew on every 80 ms tick regardless of whether anything had changed. An idle chart put a DEC 2026 open/close pair on the wire ~12.5 times a second, forever, for a picture that was not moving.

Measured: 244 repaints over a 20-second idle wait, against 2 immediately after load. Now: 0.

The fix

Draw when something changed. Three sources:

• input — a key, a mouse report, a resize;
• the full-clear flag — app.redraw implies a draw, or the clear would blank the screen with nothing to repaint it;
• a fetch landing — the one model change no event announces, so it is detected rather than reported, by watching Load::Loading leave.

The spinner is the exception that shapes the guard

ui::loading() turns off app.tick with no event behind it, so a naive "draw only on change" would freeze it mid-fetch. The guard is dirty || matches!(app.load, Load::Loading) for exactly that reason, and tick still advances every iteration so it turns at the rate it always did. This was the case I flagged as deciding whether #102 was a one-line guard or a real change — it is why the condition has two halves.

Four tests had to move, and three of them get better for it

Three used "wait for N more repaints" to mean "give it time to have acted, then check nothing did" — an idiom the free-running timer was propping up. They now wait for the picture to hold still (wait_stable), which is both what they actually meant and what survives the app getting quieter still.

The fourth is the interesting one. an_idle_chart_stops_writing counted printable characters in idle frames and asserted they were zero — the next best thing available while the frames could not be stopped. Its claim is now strictly stronger: there are no idle frames. It asserts that, and that a keystroke still draws, so "idle" cannot be confused with "wedged".

Known and accepted

today() reads the clock when --today is absent, so a chart left open across midnight no longer self-corrects on the next tick. Any key fixes it, and the alternative is a wakeup every 80 ms for the life of the process. Recorded here rather than discovered later.

240 tests pass; stress dispatched against this branch.