test(benchmarks): host-run tests write fixture executables into the live ladder evidence root via a hardcoded path

[DecisionNerd]

Problem

benchmarks/tests/test_progressive_host_run.py:31 hardcodes WORK_PARENT = Path("/home/ubuntu/graphforge-ladder"), the same directory the real progressive host ladder writes its retained evidence to (clean-<sha>-evidence/, clean-<sha>-work/, hand repro dirs). test_progressive_run.py:1308 uses the same literal path.

The tests create that directory if missing (WORK_PARENT.mkdir(parents=True, exist_ok=True), line 111), open tempfile.TemporaryDirectory(dir=WORK_PARENT) inside it, and then call write_host_bundle(output, …) with output equal to the temporary directory itself. host_run_fixture.executables(output.parent) therefore writes three fixture executables (gf, certify, generator, contents bounded executable fixture) into the live ladder root, and nothing removes them.

Observed 2026-09-21 on OVHC-AGENCY: /home/ubuntu/graphforge-ladder has birth time 18:48:27 UTC and contains only those three 26-byte fixture files. Every evidence directory that was there at 17:48 UTC (cited by #1526 and by #1478 §12) is gone. This issue does not claim the tests deleted them; the directory was absent when the tests ran and they recreated it. The defect is that a unit test suite shares a path with, and writes into, the host's retained benchmark evidence root at all.

Why the path is hardcoded

The host controller's native filesystem admission requires an ext4/xfs/btrfs volume, and /tmp on this host is tmpfs, so tmp_path cannot be used directly. That justifies an ext4-backed scratch root, not the evidence root.

Acceptance criteria

• Host-run tests take their scratch root from an environment variable (for example GF_LADDER_TEST_ROOT) with a default that is not the evidence root and is documented next to the ext4 requirement; the default must be a dedicated directory such as /home/ubuntu/graphforge-ladder-test.
• No test writes outside its own temporary directory: write_host_bundle/executables receive a path inside the temporary directory, and a test asserts the scratch root's top level is unchanged after the suite runs.
• test_progressive_run.py:1308 uses the same scratch-root mechanism; no literal /home/ubuntu/graphforge-ladder remains under benchmarks/tests/.
• docs/development/perf-g500-ladder.md names the evidence root as retained data that tests and cleanup tooling must not touch.

Non-goals

Recovering the deleted evidence (tracked in #1530) or changing the ladder controller's own output layout.