docs: cut CLAUDE.md to the rules it is for

CLAUDE.md had grown to 4,046 lines, ~3,300 of them per-component
write-ups: why a breakpoint is 500px, why a cap is a quarter, what a
spec once missed. Every session loaded all of it, and the rules that
apply to every change were buried among decisions that apply to one.

Those write-ups were already duplicated as comments beside the code
they describe -- every issue number they cite also appears in a code
comment -- so they are deleted rather than moved. What is left is the
tracker workflow, the commands, the verification tiers and a set of
broad engineering rules distilled from them, each pointing at where
its example lives. Three declined decisions recorded nowhere else go
to NOTES.md, and three code comments that cited CLAUDE.md sections by
name no longer do.

Closes #256

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/test/components/progress-line.test.ts

@@ -172,8 +172,7 @@ describe('<player-progress-line>', () => {
      * The reason this component asks `matchMedia` instead of letting a
      * stylesheet hide it: a media query cannot stop a 1 Hz interval
      * running for the life of every desktop session. That claim is
-     * load-bearing in CLAUDE.md, so it is asserted rather than
-     * described — the timer count, because a desktop render is empty
+     * load-bearing, so it is asserted rather than described — the timer count, because a desktop render is empty
      * either way and so cannot tell the two apart.
      */
     it('runs no interpolation timer above the breakpoint', async () => {