docs: refresh README, CONTRIBUTING, and examples for v0.3.0

Four short prose corrections folded together to align operator-facing
docs with current v0.3.0 reality:

- README Rust prerequisite reconciled with the toolchain pin (1.94.1,
  was 1.85+).
- CONTRIBUTING.md bumps the Rust prerequisite and adds the squash-merge
  policy note.
- examples/sidecar-nostr-relay/Dockerfile drops stale --features tui
  and the paired --no-default-features; the tui/ble/gateway cargo
  features were replaced by platform cfg gates in cbc7809.
- README Features list adds two v0.3.0-visible items: the mesh-
  interface security baseline (fips.nft conffile, fips.d/ drop-in,
  opt-in fips-firewall.service across all packaging formats) and the
  fipsctl stats time-series queries plus fipstop inline sparkline
  dashboards.

Status badge bump (v0.3.0--dev to v0.3.0) is deferred to tag time per
the release-prep checklist.

examples/sidecar-nostr-relay/Dockerfile

@@ -10,7 +10,7 @@ WORKDIR /build
 COPY Cargo.toml Cargo.lock rust-toolchain.toml build.rs ./
 COPY src ./src
 
-RUN cargo build --release --no-default-features --features tui && \
+RUN cargo build --release && \
     cp target/release/fips target/release/fipsctl target/release/fipstop /usr/local/bin/
 
 # ── Runtime stage ──