Observability: first-class 'why is this route not fast-pathing' signal #721

User-agreed valuable during the #695 shell-fast-path work, never filed. A route can silently miss the FULL fast path (handlers re-run per HIT) or shelling entirely for reasons invisible without source-diving: handlerLiveHoles (which push disqualified it and why — nested thenable, pending at putShell, handler-invoked loader), capture refusals (identity guard, Set-Cookie once #713 lands, no loading() boundary), backoff state. Proposal: a debug header variant (e.g. x-rango-shell: HIT;handlers=live;cause=nested-push@ShellHandles) gated on INTERNAL_RANGO_DEBUG, plus a once-per-key dev log naming the disqualifying push/site. Refs: #695 notes ('observability follow-up'), the eligibility taxonomy in shell-capture.ts.