fix(toolchain): invalidate target/ + sccache after a nightly bump

[githubrobbi]

Problem

After just toolchain-sync bumps the rust-toolchain.toml nightly pin, the next local build dies with:

error[E0514]: found crate `assert_cmd` compiled by an incompatible version of rustc
  = note: ... compiled by rustc 1.98.0-nightly (6bdf43094 2026-06-01)
  = help: please recompile that crate (consider running `cargo clean` first)

(also hit winnow, tempfile, toml_parser). This blocked pre-push gates this session.

Root cause — two stale caches, not one

toolchain-sync probes candidate nightlies in a per-candidate scratch CARGO_TARGET_DIR (it already knows sharing the main target/ across toolchains throws E0514), then commits the bump and exits without touching the main caches:

1. target/ — rlibs/rmeta carry the old rustc commit-hash; cargo fingerprints don't reliably force a full rebuild across a nightly bump.
2. the sccache daemon — the non-obvious one. The long-lived sccache server caches its compiler-version detection keyed by the rustc binary's mtime/size. Under rustup, ~/.cargo/bin/rustc is a toolchain-agnostic proxy (symlink → rustup) whose bytes never change across a bump, so the daemon keeps serving the OLD nightly's objects and re-poisons target/ right after a cargo clean. This is why cargo clean alone didn't fix it.

Fix

After toolchain-sync commits a bump (only in the "found newer nightly" branch, not "no bump available"), run:

• cargo clean — clear the old-nightly target/.
• sccache --stop-server — force the daemon to re-probe rustc -vV on next invocation, restoring correct cache keying. A full 30 GiB cache wipe is unnecessary; the daemon restart is the targeted fix.

Verification

• Reproduced E0514 locally; fixed with cargo clean + sccache --stop-server; full cargo build --workspace --all-targets clean afterward.
• This PR's own pre-push gate passed all Rust compilation gates (cargo-check, lint-tests, tests, lint-ci-windows) — the exact gates that were failing with E0514 before the cache fix.