Rationalize cargo feature and platform-gate surface (#79)

Drop the `tui`, `ble`, and `gateway` cargo features and replace
them with platform cfg gates. Plain `cargo build` now produces
every subsystem appropriate for the target platform with no
feature flags required.

Motivation:
- `default = ["tui", "ble"]` broke `cargo build` on macOS and
  Windows because `ble` pulled in `bluer` (BlueZ, Linux-only).
  Every non-Linux packager needed `--no-default-features`.
- The feature flags on `ble` and `gateway` were redundant with
  their platform-gated deps (`bluer`, `rustables`). The parallel
  gating was inconsistent and error-prone.
- `tui` feature protected against a ratatui binary-size concern
  that no longer applies in 2026.

Cargo.toml:
- Remove `tui`, `ble`, `gateway` features; `default = []`.
- Promote `ratatui` to a non-optional top-level dependency.
- Move `rustables` from top-level optional into the Linux
  target block, non-optional.
- Split `bluer` into its own target block with
  `cfg(all(target_os = "linux", not(target_env = "musl")))`
  — BlueZ isn't available on musl router targets and
  `libdbus-sys` doesn't cross-compile to musl without pkg-config
  sysroot setup.
- Drop `required-features` from the `fipstop` and `fips-gateway`
  `[[bin]]` entries.

build.rs:
- Emit a `bluer_available` custom cfg when `target_os == "linux"`
  and `target_env != "musl"`, for use in place of the verbose
  full predicate in source cfg gates.

Source:
- Replace every `#[cfg(feature = "gateway")]` with
  `#[cfg(target_os = "linux")]`. Gateway code works on both
  glibc and musl Linux (rustables is fine on musl).
- Replace every `#[cfg(feature = "ble")]` with
  `#[cfg(bluer_available)]`. BLE-specific code (BluerIo module,
  bluer type conversions, BLE transport instance creation,
  resolve_ble_addr) is excluded on musl and non-Linux. Generic
  `BleAddr`, `BleIo` trait, `MockBleIo`, and `BleTransport<I>`
  still compile on all targets.
- `src/bin/fips-gateway.rs`: always compiled, but `main()` is
  gated to Linux. Non-Linux stub exits 1 with a diagnostic.
  Existing non-Linux packaging scripts don't ship it, so the
  stub binary sits unused.

Packaging and CI:
- Drop `--features` and `--no-default-features` flags from every
  packaging script and workflow. Defaults now match each
  platform's capabilities.
- AUR `fips-git` automatically aligns with stable `PKGBUILD`
  (both build with defaults).

Verified: `cargo build --release` with no flags produces all
four binaries on glibc Linux; all unit and integration tests
pass across Linux/macOS/Windows/OpenWrt (musl) in CI.

src/node/mod.rs

@@ -800,7 +800,7 @@ impl Node {
         }
 
         // Create BLE transport instances
-        #[cfg(target_os = "linux")]
+        #[cfg(bluer_available)]
         {
             let ble_instances: Vec<_> = self
                 .config
@@ -810,7 +810,7 @@ impl Node {
                 .map(|(name, config)| (name.map(|s| s.to_string()), config.clone()))
                 .collect();
 
-            #[cfg(all(feature = "ble", not(test)))]
+            #[cfg(all(bluer_available, not(test)))]
             for (name, ble_config) in ble_instances {
                 let transport_id = self.allocate_transport_id();
                 let adapter = ble_config.adapter().to_string();
@@ -833,12 +833,10 @@ impl Node {
                 }
             }
 
-            #[cfg(any(not(feature = "ble"), test))]
+            #[cfg(any(not(bluer_available), test))]
             if !ble_instances.is_empty() {
                 #[cfg(not(test))]
-                tracing::warn!(
-                    "BLE transport configured but 'ble' feature not enabled at compile time"
-                );
+                tracing::warn!("BLE transport configured but this build lacks BlueZ support");
             }
         }
 
@@ -908,7 +906,7 @@ impl Node {
     /// Resolve a BLE address string (`"adapter/AA:BB:CC:DD:EE:FF"`) to a
     /// (TransportId, TransportAddr) pair by finding the BLE transport
     /// instance matching the adapter name.
-    #[cfg(target_os = "linux")]
+    #[cfg(bluer_available)]
     fn resolve_ble_addr(&self, addr_str: &str) -> Result<(TransportId, TransportAddr), NodeError> {
         let ta = TransportAddr::from_string(addr_str);
         let adapter = crate::transport::ble::addr::adapter_from_addr(&ta).ok_or_else(|| {