nostr: per-peer NAT-traversal failure suppression and clock-skew handling

Public-test daemons with populous open-discovery caches generate
sustained NAT-traversal-failure WARN volume (~140/hour, ~3500/day)
against cache-learned peers that have gone offline — their adverts
are absent from major Nostr relays but cached entries persist until
their advertised `valid_until` expires. The daemon kept publishing
offers indefinitely under exponential backoff with no per-peer
suppression, drowning operator signal and hammering relays. A
parallel concern: the strict freshness check at signal.rs silently
rejected offers under modest clock skew (now_ms() anchors to
SystemTime once at startup, so post-startup NTP step adjustments
don't propagate on long-uptime daemons), indistinguishable from
"peer is offline."

Six independent improvements layered on the existing retry logic.

Per-npub WARN log rate-limit
----------------------------
New `FailureState` struct on `NostrDiscovery` records per-npub
`last_warn_at_ms`. Subsequent failures inside `warn_log_interval_secs`
(default 5 min) emit DEBUG instead of WARN. Each WARN now also
carries `consecutive_failures` and remaining `cooldown_secs` so
operators can read the trajectory without grepping multiple lines.

Per-npub consecutive-failure counter + extended cooldown
--------------------------------------------------------
After `failure_streak_threshold` (default 5) consecutive failures
against a peer, the next `extended_cooldown_secs` (default 1800)
of attempts are suppressed by pushing
`retry_pending[npub].retry_after_ms` past the cooldown wall. The
open-discovery sweep also consults `cooldown_until` and increments
a new `skipped_cooldown` counter so a peer whose `retry_pending`
was cleared by max_retries doesn't get re-enqueued during the
cooldown window. Caps offer-publish rate per dead peer regardless
of how often the sweep tries to re-enqueue.

Stale-advert eviction on streak-threshold transition
----------------------------------------------------
On the threshold-crossing transition (one-shot, not every
subsequent failure), `tokio::spawn` an active re-fetch of the
peer's Kind 37195 advert from `advert_relays`. Three outcomes:
- absent on relays → cache evicted; sweep won't re-enqueue
  (peer is genuinely gone).
- newer `created_at` → cache refreshed + streak reset
  (peer republished; allowed to retry immediately).
- same → cache untouched; cooldown stands.

Cost: ~one fetch per dead peer per 30-min cooldown cycle, vs
hundreds of offer publishes/hour today.

Clock-skew tolerance on freshness check
---------------------------------------
`signal.rs` `validate_offer_freshness` and
`validate_traversal_answer_for_offer` now allow ±60s grace beyond
strict TTL. Both return a new `FreshnessOutcome` enum so callers
can DEBUG-log when an offer/answer was only accepted via the grace
window. `FRESHNESS_SKEW_TOLERANCE_MS` is hard-coded — loosening
this past minutes erodes the freshness/replay security boundary
and operators tend to tune in the wrong direction.

NTP-style skew estimate (offer_received_at echo)
------------------------------------------------
Added optional `offerReceivedAt: Option<u64>` field to
`TraversalAnswer` payload. Responder fills it with `now_ms()` at
offer-receipt time. Initiator computes the standard NTP offset
formula `((T2-T1) + (T3-T4)) / 2` against the round-trip and
DEBUG-logs when `|skew| ≥ 30s`. Skew is also stashed in
`FailureState` and surfaced in `show_peers`. Non-breaking — older
responders that don't fill the field still produce valid answers,
and `estimate_clock_skew` returns `None`.

Per-peer state in `show_peers` JSON
-----------------------------------
Each peer entry in `show_peers` now carries:

  "nostr_traversal": {
    "consecutive_failures": <u32>,
    "in_cooldown": <bool>,
    "cooldown_until_ms": <u64 | null>,
    "last_observed_skew_ms": <i64 | null>
  }

Always emitted (schema-stable); values populated when discovery is
enabled and the npub has a recorded entry. Required a new public
`Node::nostr_discovery_handle()` accessor and refactored
`FailureState`'s internal Mutex from `tokio::sync` to `std::sync`
(operations never hold across await), which lets the synchronous
`show_peers` handler call `snapshot()` directly without the
dispatcher becoming async.

New config knobs (under `node.discovery.nostr`)
-----------------------------------------------
  failure_streak_threshold: 5
  extended_cooldown_secs: 1800
  warn_log_interval_secs: 300
  failure_state_max_entries: 4096

Tests
-----
12 new unit tests:
- 5 in `tests.rs` covering freshness strict / tolerated / rejected
  outcomes, NTP skew estimation, and the backward-compat None case
  when the responder didn't fill `offer_received_at`.
- 7 in `failure_state.rs` covering streak/warn-rate-limit state
  transitions, cooldown active vs expired semantics,
  success-resets-streak, observed-skew records, and size-cap
  eviction by oldest `last_failure_at`.

CHANGELOG entries added under `[Unreleased]` Fixed.

CHANGELOG.md

@@ -274,6 +274,31 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Nostr-discovery now tolerates ±60s of clock skew on offer/answer
+  freshness checks so a responder whose wall clock leads the
+  initiator's by less than that no longer silently rejects every
+  offer. Previously, a public-test daemon with un-NTP'd peers (or
+  long uptime — `now_ms()` anchors to `SystemTime` once at startup,
+  then advances monotonically; post-startup NTP step adjustments
+  don't propagate) would see ~100% signal-timeout rate against
+  skewed peers, indistinguishable from "peer is offline." New
+  optional `offerReceivedAt` field on the answer payload lets the
+  initiator log per-peer NTP-style skew estimates (DEBUG when ≥30s)
+  for operator visibility. Backward-compatible — older responders
+  that don't fill the field still produce valid answers
+- Nostr-discovery NAT-traversal failure suppression: per-npub
+  consecutive-failure counter triggers a 30-min extended cooldown
+  after 5 failures, preventing the daemon from hammering Nostr
+  relays with offers to peers that have gone away. WARN log lines
+  rate-limited to one per peer per 5 min (subsequent failures
+  emit DEBUG with `consecutive_failures` + remaining `cooldown_secs`).
+  Threshold-crossing also fires a one-shot active re-check of the
+  peer's Kind 37195 advert against `advert_relays`; absent →
+  evict cache; newer → refresh + reset streak; same → cooldown
+  stands. New `failure_streak_threshold`, `extended_cooldown_secs`,
+  `warn_log_interval_secs`, `failure_state_max_entries` config
+  fields under `node.discovery.nostr`. Per-peer state visible in
+  `fipsctl show peers` JSON under `nostr_traversal`
 - Tor onion adverts published over Nostr overlay discovery now
   include the public-facing port (`<onion>.onion:<port>`) instead of
   just the bare onion hostname. The publisher previously emitted a