Migrate crates/pycc_codegen test wrapper onto ScratchDir (Batch A of #782)

[rotnov]

Summary

Batch A of the sequential migration plan published on #782: retires crates/pycc_codegen/src/tests_support.rs's tempfile_dir/TempTestDir wrapper entirely, replacing every call site with pycc_scratch::ScratchDir.

• Adds pycc_scratch as a [dev-dependencies] entry to root Cargo.toml and to crates/pycc_codegen/Cargo.toml (the latter had no [dev-dependencies] section before).
• Migrates all 273 tempfile_dir("label") call sites in crates/pycc_codegen/src/tests.rs (269 literal-argument calls plus 5 &format!(...)-argument calls migrated one at a time in the str_truthy/float_truthy/str_cmp_runtime/nested_finally/try_pep758_multi tests — one more site than initially matched by the bulk-substitution regex, brought the total to 273 as the plan estimated) to pycc_scratch::ScratchDir::new("label").expect("failed to create scratch dir").
• Migrates the file's one raw std::env::temp_dir().join(...) occurrence (write_to_file_failure_is_reported_as_an_error) to derive its path from a new ScratchDir binding instead — the test deliberately builds a nonexistent nested path to exercise a real failure mode, so no create_dir/create_dir_all was added; only the root of the path changed.
• Migrates crates/pycc_codegen/src/bigint_rc.rs's 2 call sites and drops its use crate::tests::tempfile_dir; import.
• Deletes crates/pycc_codegen/src/tests_support.rs and the #[path = "tests_support.rs"] mod support; pub(crate) use support::tempfile_dir; wiring in tests.rs.
• Shrinks scripts/check_scratch_dir_usage.py's ALLOWLIST: removes the now-zero crates/pycc_codegen/src/tests.rs entry and the entry for the now-deleted tests_support.rs. No other files' entries were touched — bigint_rc.rs was never in ALLOWLIST (0 raw occurrences to begin with, only wrapper calls, confirmed before editing).

Every ScratchDir::new(...) call in this diff uses .expect(...) only, never ? or match/if let Err, per the plan's D-014 coverage-gate guidance (§6.1) — this is a near-pure constructor swap with no new fallible branch requiring a dedicated failure-path test.

Part of #782 (Batch A). The remaining batches — src/main.rs's test module + src/project_config.rs (Batch B), and the remaining tests/*.rs integration files (Batches C/D) — are separate follow-up pull requests; #782 stays open until all batches land.

Baseline note

Branched from feat/issue-781-scratch-dir (PR #786's head), re-confirmed still open and unmerged both before starting and immediately before opening this PR, per the plan's own re-resolution instruction (§1).

Overlap note

Cross-referenced open PRs against crates/pycc_codegen/src/tests.rs and bigint_rc.rs before starting: PR #780 (feat/issue-769-optional-narrowing) appends new tests to the end of tests.rs using tempfile_dir(...) calls of its own. This is a same-file textual collision (not a logical one — different, non-overlapping regions of the file) consistent with the plan's own risk table (§7); whichever of this PR / #780 merges second will need to migrate #780's newly-added call sites onto ScratchDir as part of its own rebase, or as quick follow-up. No other open PR touches either target file.

Test plan

• cargo build --workspace --tests after the setup step (Cargo dev-dependency additions) alone, before any call-site edits
• cargo build --workspace --tests after the full batch
• cargo test --workspace — 0 failures across the full suite
• cargo clippy --workspace --all-targets -- -D warnings — clean
• python3 -B scripts/check_scratch_dir_usage.py — no violations
• python3 -B -m unittest discover -s scripts -p 'test_*.py' — 951 tests, OK (6 skipped)
• cargo llvm-cov --workspace --fail-under-lines 100 --fail-under-regions 100 — 100.00% lines, 100.00% regions
• D-068 pinned local reviewer (ievo:deep-reviewer) — 0 findings, independently re-verified the 273/273 ScratchDir::new/.expect pairing, the ALLOWLIST counts against the live tree, and the write_to_file_failure_is_reported_as_an_error nonexistent-path invariant