Disambiguate the mesh-lookup "discovery" name across the source tree

The identifier "discovery" named three unrelated subsystems; the FMP
overlay coordinate-lookup subsystem is now consistently "lookup". This
finishes the concept-#1 rename across the shell, config, and metric
layers left after the earlier proto-layer rename:

- Handler module node::handlers::discovery -> node::handlers::lookup, and
  reset_discovery_backoff -> reset_lookup_backoff.
- The Node lookup-engine field Node.discovery -> Node.lookup, renamed by
  resolved binding so the metrics().discovery and node.discovery config
  paths are left untouched.
- The lookup metric types DiscoveryMetrics -> LookupMetrics,
  DiscoveryStatsSnapshot -> LookupStatsSnapshot, and Metrics.discovery ->
  Metrics.lookup.

Two surfaces cross a stability boundary and ship behind a compatibility
window, both marked in-code for removal at the v2 cutover:

- The control-socket metric family is dual-emitted under both "discovery"
  (deprecated alias) and "lookup" so existing dashboards keep working.
- The node.discovery.* config table is split into node.lookup.* (mesh
  lookup scalars) and node.rendezvous.* (nostr/lan peer rendezvous).
  NodeConfig does not deny unknown fields, so a naive rename would make a
  deployed node.discovery: block deserialize into nothing and silently
  revert every setting to default. A deprecated all-Option
  DiscoveryConfigCompat field captures a legacy block and a new post-parse
  Config::normalize_deprecated_keys pass folds it into the new tables with
  a one-time deprecation warning.

Flip the packaged fips.yaml templates to the new keys, add legacy/new/
scalar compat parse tests, and record the split and deprecations in the
CHANGELOG. Behavior-neutral; fmt/clippy clean, lib suite green.

packaging/common/fips.yaml

@@ -10,12 +10,21 @@ node:
     #
     # Or set an explicit key (overrides persistent):
     # nsec: "nsec1..."
-  discovery:
-    # Optional Nostr-mediated overlay endpoint discovery.
+  # Mesh-lookup protocol (node.lookup.*): the overlay coordinate-lookup engine
+  # (mesh address -> coordinates). Defaults shown; uncomment to override.
+  # lookup:
+  #   ttl: 64
+  #   attempt_timeouts_secs: [1, 2, 4, 8]
+  #   recent_expiry_secs: 10
+  #   backoff_base_secs: 0
+  #   backoff_max_secs: 0
+  #   forward_min_interval_secs: 2
+  rendezvous:
+    # Optional Nostr-mediated overlay endpoint rendezvous.
     # nostr:
     #   enabled: true
     #   policy: configured_only  # disabled | configured_only | open
-    #   open_discovery_max_pending: 64 # caps queued open-discovery retries
+    #   open_discovery_max_pending: 64 # caps queued open-rendezvous retries
     #   app: "fips-overlay-v1"
     #   advertise: true
     #   advert_relays:
@@ -34,17 +43,17 @@ node:
     #     - "stun:stun.cloudflare.com:3478"
     #     - "stun:global.stun.twilio.com:3478"
     #
-    # Optional mDNS-based LAN discovery for sub-second same-LAN pairing.
+    # Optional mDNS-based LAN rendezvous for sub-second same-LAN pairing.
     # Opt-in (default false): default-off avoids a per-LAN identity
-    # broadcast on nodes that have deliberately disabled other discovery
+    # broadcast on nodes that have deliberately disabled other rendezvous
     # channels, and avoids any multicast surprise on upgrade. Requires an
     # operational UDP transport (the advertised port is the one peers dial).
     # lan:
     #   enabled: false
     #   # Optional application/network scope carried in the LAN-only TXT
     #   # record. Browsers that set a scope ignore adverts for other scopes.
-    #   # Kept separate from the Nostr discovery `app` tag so relay-visible
-    #   # adverts can stay generic while LAN discovery stays per-private-network.
+    #   # Kept separate from the Nostr rendezvous `app` tag so relay-visible
+    #   # adverts can stay generic while LAN rendezvous stays per-private-network.
     #   # scope: "lab-floor-3"
     #   # Advanced: overrides the mDNS service type. Leave unset in normal
     #   # use — only needed to run multiple isolated services on one
@@ -147,5 +156,5 @@ peers: []
   #     - transport: udp
   #       addr: "test-us01.fips.network:2121"  # IP or hostname (e.g., "peer.example.com:2121")
   #     - transport: udp
-  #       addr: "nat"  # Use node.discovery.nostr for Nostr/STUN hole punching
+  #       addr: "nat"  # Use node.rendezvous.nostr for Nostr/STUN hole punching
   #   connect_policy: auto_connect

packaging/openwrt-ipk/files/etc/fips/fips.yaml

@@ -10,12 +10,21 @@ node:
     #
     # Or set an explicit key (overrides persistent):
     # nsec: "nsec1..."
-  discovery:
-    # Optional Nostr-mediated overlay endpoint discovery.
+  # Mesh-lookup protocol (node.lookup.*): the overlay coordinate-lookup engine
+  # (mesh address -> coordinates). Defaults shown; uncomment to override.
+  # lookup:
+  #   ttl: 64
+  #   attempt_timeouts_secs: [1, 2, 4, 8]
+  #   recent_expiry_secs: 10
+  #   backoff_base_secs: 0
+  #   backoff_max_secs: 0
+  #   forward_min_interval_secs: 2
+  rendezvous:
+    # Optional Nostr-mediated overlay endpoint rendezvous.
     # nostr:
     #   enabled: true
     #   policy: configured_only  # disabled | configured_only | open
-    #   open_discovery_max_pending: 64 # caps queued open-discovery retries
+    #   open_discovery_max_pending: 64 # caps queued open-rendezvous retries
     #   app: "fips-overlay-v1"
     #   advertise: true
     #   advert_relays:
@@ -121,5 +130,5 @@ peers: []
   #     - transport: udp
   #       addr: "test-us01.fips.network:2121"  # IP or hostname (e.g., "peer.example.com:2121")
   #     - transport: udp
-  #       addr: "nat"  # Use node.discovery.nostr for Nostr/STUN hole punching
+  #       addr: "nat"  # Use node.rendezvous.nostr for Nostr/STUN hole punching
   #   connect_policy: auto_connect