Stop hook: scope clippy + tests to affected crates instead of --workspace

[zackees]

Problem

ci/hooks/check-on-stop.py runs cargo clippy --workspace --all-targets and cargo test --workspace on every Stop where any file changed. Even one-line edits in fbuild-core rebuild + retest all 11 crates' worth of tests, benches, and binaries.

Locally measured: ~5+ minutes for the full workspace test compile + run, ~2-3 min for workspace clippy, sharing the same target/ so they serialize on the cargo lock anyway.

Fix

Diff git diff --name-only HEAD --diff-filter=ACMR against crates/<name>/... and run scoped commands:

```python
changed = git_diff_names()
crates = set()
needs_full = False
for path in changed:
if path.startswith("crates/"):
crates.add(path.split("/")[1])
elif path in ("Cargo.lock", "Cargo.toml", "rust-toolchain.toml"):
needs_full = True
elif path.startswith(".cargo/"):
needs_full = True

if needs_full:
run("soldr cargo clippy --workspace --all-targets -- -D warnings")
run("soldr cargo test --workspace")
else:
for c in crates:
run(f"soldr cargo clippy -p {c} --all-targets -- -D warnings")
if crates:
args = " ".join(f"-p {c}" for c in crates)
run(f"soldr cargo test {args}")
```

(Same logic the existing ci/lint.py:detect_crate already encodes for single-file edits — generalize it to multi-file.)

Why workspace-wide should stay in pre-push / CI

Workspace-wide guarantees we don't break a cross-crate interaction; that's catchable on push to a remote where CI also runs the same matrix. Stop hook is per-conversation interactive feedback — it should match what the user can read in <60s.

Acceptance

• Single-crate .rs edit + Stop: hook runs cargo clippy -p <crate> + cargo test -p <crate> only. <60s warm.
• Cargo.toml / Cargo.lock / toolchain change: hook falls back to --workspace. (Same behavior as today.)
• pre-push hook (new or augmented) runs the full --workspace lint + test so escapes are still caught before share.

Related

• Subtask of Stop hook ~5-10 min on every Stop; needs structural cut #462 (umbrella: Stop hook is too slow).
• Quick-win prereq: Stop hook: add PEP 723 header to check-on-stop.py to skip uv sync #464 (PEP 723 fix for check-on-stop.py).