fix(lore-0521): the seed writes signers only for accounts newer than ours

[karolko9]

Logic, ops tool (backfill-runner snapshot-seed) — task 0521. The seed writes signers only for accounts newer than ours. Replaces #446.

Why

Pass 4 of snapshot-seed emitted every live account into account_entry_state on every run. That was right before the live signers writer existed and wrong since it deployed (2026-08-24): the second production pass re-emitted 10,872,072 rows identical to what we held. The data was never at risk (same version, RMT collapses them), but the summary line was a constant and could not show the live writer stopping.

#446 fixed this on the old seed.rs layout; task 0210 has since split the seed passes into modules, so the gate is rebuilt on the new structure.

What changes

• snapshot/entry_state.rs (new, beside balances / claimable / pools): reads our newest last_updated_ledger per account (sliced on account_id, RMT duplicates grouped) and writes a row only when the snapshot's entry is newer — pools::need, the rule the classic-pool pass already applies.
• seed.rs: pass 4 calls the module (−15 lines); summary.txt reports account_entry_state N (a new, b changed; c unchanged), the three summing to the live accounts.
• docs/backfills.md: how to read that line — a later pass that writes millions again means the live signers writer stopped stamping.

Verified

• backfill-runner tests: 84 pass, incl. new only_accounts_newer_than_ours_are_written (new, behind, same version, ours ahead, merged-away account); with the gate removed it fails ([1, 2, 3, 4] written instead of [1, 2])
• clippy -D warnings, fmt clean
• Production dry run (read-only, 2026-09-30, checkpoint 64,699,071, 1,339 s): account_entry_state 0 (0 new, 0 changed; 11004932 unchanged) — the three sum to the snapshot's 11,004,932 live accounts exactly; we hold 11,130,680. Every other correction as expected: 17 balance ghosts (16 classic, 1 native, 0.0061841 XLM), no stubs, no unresolved references.
• Unrelated counter seen in the same run: native divergent SAME ledger 16,511 (18,363 on 2026-09-02) — task 0514, not touched here.

Deploy

None — an operator tool run by hand.

Refs task 0521.