Address review round 2: preserve delta insert arity; reject unpinned header-deltas

Residuals from the Codex re-review:

1. A system delta's insert was flattened to one token, so deltas differing
   only in inserted-line count compared equal. Now one {{system}} token per
   inserted line — position AND extent survive, content does not.

2. The live uniformity guard folded only request/header snapshots, so a
   mid-run header CHANGE (request/header-delta) could diverge from the pin
   invisibly. Non-pinning runs now assert zero header-delta events: a
   scenario that legitimately changes its header mid-run exists to show
   that change, so it must pin (fail-loud until it does).

examples/acp-agent/tests/snapshot-normalize.spec.ts

@@ -135,13 +135,14 @@ describe('scrubRequestHeaders', () => {
     expect(out).not.toContain('{{tools}}')
   })
 
-  it('scrubs a header-delta system payload but keeps its line positions', () => {
+  it('scrubs a header-delta system payload but keeps its line positions and arity', () => {
     const delta = JSON.stringify({
       type: 'request/header-delta', seq: 8, time: 9,
-      data: { system: { keepStart: 1, keepEnd: 4, insert: ['leaked prompt line'] }, config: { model: 'm2' } },
+      data: { system: { keepStart: 1, keepEnd: 4, insert: ['leaked prompt line', 'second line'] }, config: { model: 'm2' } },
     })
     const out = scrubRequestHeaders(`${headerLine}\n${delta}\n`)
-    expect(out).toContain('"insert":"{{system}}"')
+    // One token PER inserted line: the edit's position AND extent survive.
+    expect(out).toContain('"insert":["{{system}}","{{system}}"]')
     expect(out).toContain('"keepStart":1')
     expect(out).toContain('"keepEnd":4')
     expect(out).toContain('"config":{"model":"m2"}')