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.

.github/workflows/ci.yml

@@ -82,7 +82,7 @@ jobs:
             ${{ runner.os }}-cargo-
 
       - name: Build
-        run: cargo build --release ${{ (runner.os == 'macOS' || runner.os == 'Windows') && '--no-default-features --features tui' || '--features gateway' }}
+        run: cargo build --release
 
       - name: SHA-256 hashes (Linux)
         if: runner.os == 'Linux'
@@ -147,7 +147,7 @@ jobs:
         uses: taiki-e/install-action@nextest
 
       - name: Run unit tests
-        run: cargo nextest run --all --profile ci --features gateway
+        run: cargo nextest run --all --profile ci
 
       - name: Publish test report (Checks tab)
         uses: dorny/test-reporter@v2
@@ -197,7 +197,7 @@ jobs:
         uses: taiki-e/install-action@nextest
 
       - name: Run unit tests
-        run: cargo nextest run --all --profile ci --no-default-features --features tui
+        run: cargo nextest run --all --profile ci
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Job 2c – Unit tests (Windows)
@@ -226,7 +226,7 @@ jobs:
         uses: taiki-e/install-action@nextest
 
       - name: Run unit tests
-        run: cargo nextest run --all --profile ci --no-default-features --features tui
+        run: cargo nextest run --all --profile ci
 
 # ─────────────────────────────────────────────────────────────────────────────
 # Job 3 – Integration tests (static mesh + chaos simulation)

.github/workflows/package-linux.yml

@@ -90,7 +90,7 @@ jobs:
         run: cargo install cargo-deb --version 3.6.3 --locked
 
       - name: Build release binaries
-        run: cargo build --release --features gateway
+        run: cargo build --release
 
       - name: Build systemd tarball
         env:

.github/workflows/package-macos.yml

@@ -86,7 +86,7 @@ jobs:
             macos-release-${{ matrix.arch }}-
 
       - name: Build release binaries
-        run: cargo build --release --target ${{ matrix.target }} --no-default-features --features tui
+        run: cargo build --release --target ${{ matrix.target }}
 
       - name: Build macOS package
         run: |

.github/workflows/package-windows.yml

@@ -77,7 +77,7 @@ jobs:
             windows-release-x86_64-
 
       - name: Build release binaries
-        run: cargo build --release --no-default-features --features tui
+        run: cargo build --release
 
       - name: Build Windows package
         shell: pwsh