test: fail closed when fixture temp-root allocation fails

[Chris0Jeky]

Summary

• validate both mktemp -d forms before deriving any fixture child path
• fail with setup exit 2 when neither allocation form returns a non-empty directory
• make the cleanup trap no-op when fixture-root allocation failed
• add fixture-local executable probes for fallback success and total allocation failure

Verification

• bash -n scripts/worktree_guard.tests.sh — passed
• bash scripts/worktree_guard.tests.sh — 38 passed, 0 failed, 0 skipped
• git diff --check — passed

The failure probe asserts that no primary, bare-repository, or empty-root fixture paths are created or named after both allocation forms fail. No user machine configuration or global cleanup was changed.

Refs #1855

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[Chris0Jeky]

Fresh-context review record (coordinator, 2026-09-06; read-only Opus 5 reviewer over the diff against the merge base; route: the standing D-12 rule for Codex-session PRs — review, fix MEDIUM+ in-PR, merge, disposition comment).

VERDICT: SHIP, nothing fixed in-PR. The destructive-path checks come back clean: every rm -rf -- "$FIXTURE_ROOT" is fenced by empty/not-a-directory guards with -- and no trailing slash, the trap early-returns on an empty root, and the old FIXTURE_ROOT="" path really would have run git init "/primary" at the filesystem root, so the fix is real; the failure path exits non-zero through FAIL. LOWs recorded for #1855: three new knobs undocumented in the usage header; an ambient WT_GUARD_ALLOCATE_ONLY makes the suite exit 0 silently having run zero cases; a non-numeric value prints integer expression expected; a noexec temp mount would fail both probes spuriously (hypothesis; no CI caller). Hosted Windows red at the previous head was the #2378 launcher-suite class; the branch is updated past #2608.