diigo-cache: diigo counts pages from 0; migrate stored pages (schema v2)

Comparing a cache with diigo's RSS export: the size-100 walk had missed
exactly the 100 newest bookmarks (everything updated since 2026-09-16).
diigo's page_num is 0-based, and we sent our 1-based page number
unchanged, so "page 1" was really page 2 and the newest page was never
fetched. diigo's total (16744) was right; the "total-overstated"
reading was a misdiagnosis.

- page() asks for page_num = n - 1
- BREAKING for stored state, agreed beforehand, migrated in place:
  schema v2 renumbers every stored page +1 (pages and page_members, via
  negation so no number collides), once, recorded as kv "schema" = 2;
  nothing is refetched, page 1 just becomes a miss
- fill jobs record their numbering; a job saved without it fails ("page
  numbering changed …; start the fill again") instead of resuming
- the fake pages from 0 like diigo (7 tests failed until the fix);
  tests feed v1-shaped storage and a v1-numbered job to the new code
- the next-page peek guard stays; its comments no longer claim diigo
  overstates its total
- the fixed test clock moves to 2099: pinned to "today" its alarms came
  due in real time and fired on their own

Generated-by: Claude Opus 5.5

diigo-cache/tests/fake-webapp.js

@@ -14,7 +14,8 @@ const TOTAL = 250;
 const CAPPED_WEBAPP = "fake-capped-webapp";
 const CAP = 64;
 /** the plain account, but its `total` claims 60 more items than it has
- * (as diigo's does, observed 2026-10-07) */
+ * (hypothetical: what looked like this on 2026-10-07 was the 0-based
+ * page_num, but the guard stays) */
 const OVERSTATED_WEBAPP = "fake-overstated-webapp";
 const OVERSTATEMENT = 60;
 
@@ -93,7 +94,9 @@ class FakeGet extends RpcTarget {
 		const total = this.#dated === "deep" ? DEEP_TOTAL : dated ? DATED_TOTAL : TOTAL;
 		const now = Date.now();
 		const items = [];
-		for (let n = (page - 1) * count + 1; n <= Math.min(page * count, total); n += 1) {
+		// diigo's page_num counts from 0 (observed 2026-10-07: page_num=1
+		// skipped the newest page)
+		for (let n = page * count + 1; n <= Math.min((page + 1) * count, total); n += 1) {
 			items.push(dated ? datedItem(n, now, this.#dated === "deep" ? "deep_dee" : "dated_dan") : item(n));
 		}
 		// past the end, diigo answers no items and a total of 0 (observed