chore(deps): strip phantom bincode dep, closes RUSTSEC-2025-0141

The `bincode = { version = "1.3", optional = true }` declaration in the
root `Cargo.toml` (with a `fast-serde = ["bincode"]` feature flag) was
never imported in any `.rs` file. Removing both the dependency and the
dead feature flag (plus its mention in `full`) closes
RUSTSEC-2025-0141 (`bincode@1.3.3`, advisory: bincode 1.x maintenance
sunset) at the source — no behavioural change because nothing in the
crate ever invoked the bincode API.

Verification:

    $ grep -rE 'bincode' --include='*.rs' .   # (no matches)
    $ cargo update
    $ grep -c '^name = "bincode"$' Cargo.lock # 0

`cargo check --lib` is green (4 pre-existing bin errors in src/main.rs
are unrelated — they reference `Orchestrator` methods that don't exist
on main; out of scope for this CVE strip).

Closes #33.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath