fix(controller): stop leaking test tempdirs (closes #273)

[WaylandYang]

Closes #273.

The leak

test_state and the two branch_slot_* tests in crates/forkd-controller/src/http.rs each built their scratch directory like this:

let td = tempfile::TempDir::new().unwrap();
// Leak the TempDir so it survives the test (Drop deletes the dir).
std::mem::forget(td);

It keeps the directory alive, but Drop never runs — so every cargo test -p forkd-controller orphans one directory per construction in $TMPDIR. The issue reported ~10 per run; on today's test count it is 56.

The fix

Bind the TempDir to the state rather than leaking it — the "cheapest" option from the issue. AppState gains a #[cfg(test)] pub _tempdir: Option<tempfile::TempDir> field, declared last so it drops after registry. Dropping the final Arc now reaps the directory.

This mirrors the _td: TempDir ownership pattern that tests/http_integration.rs already uses, so the unit tests and the integration tests handle scratch dirs the same way.

Two cleanups carried along:

• The duplicated inline AppState literals in branch_slot_global_cap_blocks and branch_slot_capacity_recovers_on_drop collapse into a shared test_state_with_cap(cap); test_state() delegates to it. Net −24 lines, and there is now one place to update when AppState grows a field.
• prewarm_scratch_dir moves off the shared $TMPDIR/forkd-test-prewarm into the per-test tempdir, so nothing the daemon writes escapes it.

Verification

Run on Linux, using the reporter's own measurement (ls -d /tmp/.tmp* | wc -l after a cargo test -p forkd-controller in a fresh container):

	tests	leftover /tmp/.tmp*
before	71 + 7 passed	56
after	71 + 7 passed	0

Full CI matrix green locally: cargo fmt --all -- --check, cargo clippy --all-targets --all-features -- -D warnings (no warnings), cargo build --all, cargo test --all.

Notes

• No CHANGELOG entry — this is test-only hygiene with no user-facing effect.
• #[cfg(test)] only applies to the lib crate's own unit-test build, so the field does not exist for integration tests or downstream consumers; the single production construction site in lib.rs gets a matching #[cfg(test)] _tempdir: None.
• Swept the rest of the tree for the same class of leak: no remaining mem::forget, ManuallyDrop, TempDir::into_path(), or .keep() anywhere in crates/.

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