Reduce repeated lock matrix save checks

[noeltock]

Problem

The lock-mode matrix repeats the same edit, save, parse and validation sequence four times, although the distinct behavior under test is whether each mode permits insertion, removal and movement.

Solution

Keep the persistence round trip for contentOnly and all structural operation checks for all four modes. Forwarding, rendered layout and regeneration coverage stays in place.

Behavior	Read first	Proof
Native edited content survives serialization and reopening	dev/test/authoring.runtime.test.ts	Representative contentOnly round trip
Each lock mode keeps its distinct insert/remove/move policy	Same file, lock-mode matrix	All four matrix cases retained

Diff

+13 −11 · 1 test file · no application changes

 lock-mode matrix
-  edit → serialize → parse → validate for every mode
+  edit → serialize → parse → validate for contentOnly
   insertion, removal and movement for every mode

Testing & verification

Reviewed revision: 0834cfa9879663baae5dccbc3f084c1668037269 · Node 22.23.1

• npm test -- dev/test/authoring.runtime.test.ts: 10 passed.
• npm run typecheck: passed.
• git diff --check: passed.

Exact-head CI run 38293548692 passed on Node 22.13 and 24, including full verification, package consumers and the WordPress proof job.

Not verified: runtime saving was not measured; the test case count is unchanged.

Risk / rollout

The save round trip no longer covers its cross-product with unlocked, insert and all modes. Their distinct structural checks remain. Detection: retained persistence, forwarding and regeneration assertions. Rollback: revert this test-only commit.