feat(sync): apply in pages, one frame and one rebase each (T13193 R-4c)

[kryptobaseddev]

Task: T13193

Second part of the fourth slice of the scoped rebase (journal spec §3.5 Rule 3, R6-4). Stacked on #1922 (T13273). Also closes T13274, #1921's LOW.

What changes

• Pages.
  • applyStagedTxns now groups staged txns into pages with takePage: up to REBASE_PAGE_OPS (2,000) ops, with pageOps/pageMs overrides.
  • A page breaks only between txns. A split txn is one staged unit, so its part-set stays whole. A page also breaks on an actor change, because one apply frame records one actor. A txn larger than a page is a page alone (the declared exemption).
  • The time bound (REBASE_PAGE_MS, 50 ms) stops a page from taking more txns; whatever is left starts the next page.
• One frame per page. Each page is one apply frame:
  1. planPageRebase builds the scope over the page's foreign footprints and its own echoes that fail the fast path, to a fixed point;
  2. one rewind;
  3. applyInPage runs per txn: pre-checks, planning, clock, ops in their own Gate C savepoint, post-apply checks, sequencing, inbox mark;
  4. one replay of the rewound locals whose echo the page did not decide.
• Own echoes. A rewound local txn whose echo is in the page applies at its stream position and is sequenced there (outcome applied or void). A refused one stays rewound, since the rollback is per txn. An in-place echo voided by Gate C still gets its own rewind (planOwnRebase, which leaves out what the page already rewound).
• Fast path judged once per page. echoInPlace reuses the page's unsequencedLocalTxns, and the verdict is reused at sequencing. That covers review-hotfix's O(backlog) LOW from feat(sync): scoped rebase frame - rewind, apply, replay (T13193 R-2) #1916.
• Natural twins (D2). A local insert the stream also made is rewound to the stream's row, from its replay snapshot, instead of deleted. Before, a later stream U of that row waited forever.
• T13274. A U setting part of a composite UNIQUE key fills the rest from the current row, as wire values with refs as uids.
• Nit from fix(sync): apply finds natural-key rows whose uid was never filled (T13273) #1922. Added a comment on adoptNaturalRow's === 1.

Tests

• Page tests (new, real replicas):
  • one txn per page (pageOps: 1) converges with one page, checked through applied_frame;
  • a Gate C void inside a page rolls back only that txn, its sibling applies in the same frame, and the replicas converge;
  • the time bound ends a page between txns;
  • an actor change breaks a page, and a 2-op txn under pageOps: 1 applies whole.
• Natural twin: a relation both replicas inserted, then a stream update of it, converges.
• T13274 unit tests: a partial key U, a ref part converted to a uid, and an unmoved key that adds nothing.
• Existing tests updated to page semantics:
  • the LOW-1 test now expects rebased: 2 (B's insert and A's echo share one rebased page);
  • one sequencing test's foreign touch now targets a row the stream knows, instead of an unsequenced local insert.
• Mutation checks: page ops bound, actor break, time bound, the replay excluding decided echoes, the per-page fast-path verdict, the twin rewind, and all three T13274 pieces. All killed.

Locally green:

• 264 targeted tests (apply/, repair, sealer, apply-intent, trigger-suspend, identity-fill, legacy-groups, show);
• tsc;
• biome on every file changed vs main;
• gates 3, 4, 28, 36, 37, 38 (--base origin/fix/T13273-natural-key-apply) and 40.

Not here: the R6-4 7-day backlog benchmark on a cleocode scratch copy (heavy; waiting on team-lead's call on a separate task).

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[kryptobaseddev]

Merging at the approved head 021c582 (review-hotfix APPROVE merge-only 021c582 (clean main merge over 0e8c3eb); CI: 105 checks, all green).