test(dl-reconcile): pin delimiter shape and comment indent (RIG-3944)

[rigel-mintaka]

This PR is part of a stack containing 3 PRs:

1. main
2. fix(dl-reconcile): classify ledger lines once for both counters (RIG-3634) #1327
3. fix(dl-reconcile): classify ledger lines once for both counters (RIG-3634) #1325
4. "test(dl-reconcile): pin delimiter shape and comment indent (RIG-3944)" (this PR)

Two mutations survived the suite: loosening isLedgerSeparator to a bare 4-cell check, and relaxing the comment opener to any indent. Each now fails exactly one new test.

• A 4-cell row under the header that is not a delimiter row yields no parser rows against a floor of 1.
• A comment opener indented four spaces is indented code, so the row it wraps stays content: parser 1, floor 2.

Review found two more mutations on the same checks that let the counters agree silently on a phantom row: tightening the opener to /^<!--/ and dropping the 4-cell count. Both are now pinned.

• A 3-space comment opener still hides the parked row: parser 1, floor 1.
• A 3-cell delimiter row under the 4-column header anchors no table: parser 0, floor 1.

Each of the four mutations fails only its own test (45 pass / 1 fail). All expectations were checked against GitHub's GFM renderer.

Refs RIG-3944

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-3944

[github-actions]

Compass engineering docs preview: https://compassdlreconcile-rig-3944.compass-eng-docs.pages.dev

Deployed from CompassDlReconcile/rig-3944-pin-mutations at cbfe950.