splash: give the logo a placement id so re-anchoring replaces, not stacks

A kitty placement is identified by (image id, placement id); an a=p with no
p= key is anonymous and every one stacks another copy. The splash re-anchors
on every progress note (~5/s), so a 60s load ended with ~300 placements of an
RGBA logo alpha-compositing over each other -- soft edges creeping to solid,
and the terminal re-rendering all of them per frame.

Same pair every time (LOGO_PLACEMENT) = the terminal REPLACES the placement,
so re-anchoring is free and atomic. That is also why on_resize no longer
clear()s first (that showed a hole for a frame), and why a shrunk-to-nothing
region now drops the placement instead of leaving a stale one anchored.

tests/test_kittygfx.py pins the escapes (pure, stdlib);
experiments/splash_place_count.py counts what a real splash sends.

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