[Test] Require executable lane-error anchor

[Chris0Jeky]

Summary

• make the lane-error rethrow check use PowerShell AST executable statements rather than raw text search
• add a negative control proving comments and string literals cannot satisfy the anchor
• keep the production guard and all lane behavior unchanged

Verification

• pwsh -NoProfile -ExecutionPolicy Bypass -File scripts/agentic/Test-Assert-TaskdeckCheckoutFingerprint.ps1 — 30 passed, 0 failed
• git diff HEAD^ HEAD --check — passed

Refs #1860

[chatgpt-codex-connector]

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[Chris0Jeky]

Review gate (Codex review credits exhausted, SC-9; D-12 ruled 2026-09-05: the Codex session's PRs are reviewed and merged by the coordinator): one fresh-context read-only reviewer (Opus 5) on head 2b76df430. Verdict: SHIP.

Checked by reading: Invoke-TaskdeckGuardedLane.ps1:153 is throw $laneError, the only occurrence in the repo, and a ThrowStatementAst extent starts at the keyword, so the single-quoted '^\s*throw\s+\$laneError\s*$' regex matches it; $rethrown[0] cannot fault under strict mode because -and short-circuits on the count check; offsets are comparable with the surrounding IndexOf results (same Get-Content -Raw string); the negative control is meaningful against the anchor being replaced (a comment and a string literal both satisfied the old IndexOf and yield zero throw nodes); Assert-True emits nothing on success and List[int].Add returns void, so the helper's pipeline output is clean; the harness is a required check on Windows PowerShell 5.1, so the green run proves ParseInput plus FindAll in the CI host; only the test harness is touched.

Findings and disposition (both LOW, tracked on #1860):

• the parse assertion message is hardcoded to "synthetic guard source did not parse" but the helper also parses the real production file.
• the negative control exercises the helper against synthetic text only; the production anchor is still carried by the pre-existing ordering test.

Issue disposition: plausibly completes the executable lane-error anchor row of #1860.