…16)

Closes #8.

Migrates `mlxcel-core`'s `TokenLanguageIndex` on-disk cache off the
unmaintained `bincode` 1.x (RUSTSEC-2025-0141 — project permanently
ceased development) to `postcard` 1.x.

## Architectural choice — postcard

- Drop-in serde-based API (`postcard::to_allocvec(&v)?` /
  `postcard::from_bytes::<T>(&bytes)?`) — minimal call-site change.
- 1.0+ stable with frozen wire format — protects production users'
  cache compatibility across future dependency upgrades.
- Embedded Rust ecosystem standard — natural fit for an
  inference-runtime cache.

Alternatives considered: `bitcode` (faster but pre-1.0, wire-format
changes between minors), `rkyv` (zero-copy over-engineering for a
startup-time cache), `bincode` 2.x / 3.x (different maintainer team,
name-confusion risk — Dependabot's auto-PR #10 to bincode 3.x was
explicitly closed in favor of this approach).

## Cache compatibility — graceful degradation

`lang_analyzer/cache.rs` already has corrupt-detection: a
deserialization failure renames the existing bincode-format file to
`*.broken.<epoch>.bak` and rebuilds the cache from source. Users
upgrading from a prior mlxcel release will trip this path
automatically on the first cache read — no explicit migration code
needed.

## Scope (3 files + Cargo.lock)

- `src/lib/mlxcel-core/Cargo.toml` — `bincode = "1"` → `postcard = { version = "1", features = ["alloc"] }`
- `src/lib/mlxcel-core/src/lang_analyzer/cache.rs` — 3 call sites + doc comments
- `src/lib/mlxcel-core/src/lang_analyzer/mod.rs` — error variant `Bincode` → `Postcard` (no external match-arms on it, safe rename)
- `deny.toml` — RUSTSEC-2025-0141 `[advisories.ignore]` entry removed (no longer applicable)

Verified locally: `cargo deny check` clean (advisories ok, bans ok,
licenses ok, sources ok); `cargo tree -p mlxcel-core -i bincode`
returns "package not found" — bincode is fully removed from the
dependency tree.

Self-hosted clippy + test gate is queued behind the PR #14 cold-build
backlog at merge time; will run on main after the runner clears.