Drop the nostr-discovery cargo feature flag

Make Nostr-mediated overlay discovery unconditional, mirroring the
philosophy of PR #79's collapse of the tui/ble/gateway features in
favor of platform cfg gates. nostr / nostr-sdk are pure Rust over
WebSockets/TCP, so they build cleanly on every FIPS-supported
platform — there is no need for the parallel feature gate.

The flag was already in `default = [...]`, so no behavior change for
anyone using `cargo build` without `--no-default-features`. Operators
who explicitly disabled the feature will now find Nostr code present
in the binary; the runtime check `node.discovery.nostr.enabled` still
controls whether the runtime starts.

Cargo.toml:
- Remove the `[features]` table entirely.
- Drop `optional = true` from `nostr` and `nostr-sdk`.

Source: 27 cfg sites collapsed across 5 files —
`src/discovery.rs`, `src/discovery/nostr/mod.rs`,
`src/node/handlers/rx_loop.rs`, `src/node/lifecycle.rs`,
`src/node/mod.rs`. Two `#[cfg(not(feature = "nostr-discovery"))]`
fallback blocks (the udp:nat-without-runtime debug-log path and the
"feature not compiled in" warning at startup) were removed as dead
code; the always-on path already handles the missing-runtime case
via `nostr_discovery: Option<NostrDiscovery>`.

Packaging and tooling:
- `packaging/openwrt-ipk/Makefile`: drop a stale `--features gateway`
  flag (the `gateway` feature was already removed in PR #79; this
  was a leftover that the build path tolerated only because cargo
  ignored unknown feature names).
- `testing/scripts/build.sh`: drop `DEFAULT_CARGO_BUILD_ARGS=(--features
  nostr-discovery)`; defaults are empty.
- `packaging/common/fips.yaml`: drop the "requires the
  nostr-discovery feature" comment from the discovery section.

Bundled cleanup:
- Apply `cargo clippy --fix` against three pre-existing warnings in
  `src/discovery/nostr/runtime.rs` and `src/discovery/nostr/stun.rs`
  (collapsed `if let Some` chain; two redundant `as i32` casts).
  These were always present but masked when the feature gate was
  off; they surface now that the code is unconditionally compiled.
- `cargo fmt` settled two minor formatting drift sites in
  `src/bin/fips-gateway.rs` and `src/config/mod.rs`.

Tests: 1083 passed, 0 failed, 4 ignored. clippy clean. fmt clean.

src/node/mod.rs

@@ -431,7 +431,6 @@ pub struct Node {
     retry_pending: HashMap<NodeAddr, retry::RetryState>,
 
     /// Optional Nostr/STUN overlay discovery coordinator for `udp:nat` peers.
-    #[cfg(feature = "nostr-discovery")]
     nostr_discovery: Option<Arc<crate::discovery::nostr::NostrDiscovery>>,
     /// Per-peer UDP transports adopted from NAT traversal handoff.
     bootstrap_transports: HashSet<TransportId>,
@@ -596,7 +595,6 @@ impl Node {
             ),
             pending_connects: Vec::new(),
             retry_pending: HashMap::new(),
-            #[cfg(feature = "nostr-discovery")]
             nostr_discovery: None,
             bootstrap_transports: HashSet::new(),
             last_parent_reeval: None,
@@ -724,7 +722,6 @@ impl Node {
             discovery_forward_limiter: DiscoveryForwardRateLimiter::new(),
             pending_connects: Vec::new(),
             retry_pending: HashMap::new(),
-            #[cfg(feature = "nostr-discovery")]
             nostr_discovery: None,
             bootstrap_transports: HashSet::new(),
             last_parent_reeval: None,