fix(archon): guard migrate slices, cap QUIC admission, reset backoff, fix underrun warning

[forkwright]

Four archon defects (deep-audit).

• panic: non-ASCII filenames crash podcast migration mid-run #545 — parse_date_stem sliced &stem[..10]/[..8] by byte length only; a multibyte UTF-8 char at the boundary panicked inside the spawn_blocking migration, aborting the run. Now char-boundary-guarded (returns None, no panic).
• DoS: unbounded QUIC connection admission in render server #546 — the QUIC render accept loop spawned one task per connection with no cap and no pre-auth deadline on 0.0.0.0. Now bounded by a Semaphore (max_connections, refuses beyond it) with the permit held for the connection lifetime, and a session_init_timeout drops peers that never send SessionInit.
• reconnect backoff: never resets after a healthy connection #547 — the reconnect ExponentialBackoff ratcheted to max for the process lifetime; it now reset()s after a successful session negotiation, so a healthy renderer restores initial backoff for the next transient blip.
• diagnostic: underrun warning compares counter against itself, never fires #553 — the underrun warning compared the counter against a second load of itself (never fired). Now compares against a stored last-reported value; a u64::MAX sentinel makes the first report establish the baseline without a spurious warn.

Gate green; 8 new tests (multibyte-no-panic, admission-cap-refuses, stalled-peer-dropped, backoff-reset-on-success, underrun-fires-on-increase/not-on-unchanged).

Closes #545
Closes #546
Closes #547
Closes #553