build on current Rust, clean up clippy/fmt, and add GitHub Actions CI

[asiniscalchi]

The crate no longer built on a current stable toolchain. This PR fixes the build,
resolves every clippy finding (a few of which were real bugs), formats the codebase
with rustfmt, and adds a CI workflow that keeps all of it enforced.

Build fixes

• Remove the stale [[bin]] hello entry from Cargo.toml — src/hello/ was deleted
  long ago and the dangling entry made every cargo invocation fail target resolution.
• Fix two dangerous_implicit_autorefs hard errors in audio.rs (deny-by-default on
  recent toolchains) by making the reference through the shared-memory pointer explicit.
• Rename .cargo/config → .cargo/config.toml (deprecated name, contents unchanged).

Real bugs surfaced by clippy

• Missing NUL terminators: the light, pressure, and all IMU shm/semaphore name
  constants in ceti.rs were built from byte literals without a trailing \0, so
  shm_open/sem_open would read past the end of the literal. All C-string constants
  now use c"..." literals, which are NUL-terminated by construction.
• Ignored read length in the TCP command handler (subscription.rs): commands were
  parsed from the full 1500-byte buffer including uninitialized tail; now only the bytes
  actually received are parsed.
• Unchecked write calls in the example client replaced with write_all, and an
  always-false size <= 0 comparison on an unsigned value fixed.

Cleanup

• cargo clippy --fix for the mechanical findings (unneeded returns, div_ceil,
  needless borrows) plus removal of dead code and dead stores.
• #[allow(clippy::too_many_arguments)] on tcp_handler (one subscriber list per
  sensor stream) and a SubscriberList type alias in fwd_thread.rs.
• cargo fmt over the whole codebase (whitespace-only; the large subscription.rs
  diff is rustfmt re-nesting the match arms).
  ## CI (.github/workflows/ci.yml)

Two jobs on push to main and on PRs:

1. Build & test (x86_64) — cargo fmt --check, build all targets (installs
   libasound2-dev for the rodio example), cargo clippy --all-targets -- -D warnings, cargo test.
2. Cross-compile daemon (aarch64) — release build for the whale tag using Ubuntu's
   gcc-aarch64-linux-gnu (linker overridden via env var, taking precedence over
   .cargo/config.toml), with the resulting daemon-aarch64 binary uploaded as a workflow artifact on every run.