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>

main.go

@@ -64,8 +64,7 @@ var frontendDistAssets embed.FS
 // Returning early is not a degraded mode: `nativeInit` has already
 // re-attached the bridge, so the recreated activity's WebView talks to
 // the app that is still running, with its queue and its playback
-// position intact. See CLAUDE.md, "An activity is a view onto the
-// process".
+// position intact.
 //
 // It is inert off Android, where a process has exactly one main().
 var mainStarted atomic.Bool