feat(indexes): stake address log for O(page) account address queries

[slowbackspace]

Stacked on #1221 (the base branch). Follow-up to the performance concern flagged there.

Problem

With the correct first-appearance ordering from #1221, GET /accounts/{stake_address}/addresses?order=desc must scan the account's entire archive history before it can reverse the list. Measured on a fully synced mainnet snapshot: 305 seconds (cold and warm — the cost is CPU-bound block decoding) and ~274MB transient RSS for an exchange account with 400k+ addresses. Any desc page on such an account pays the full scan, which is also a DoS shape on public nodes.

Design

A stake address log: each (stake credential, address) pair stored once, at its first on-chain appearance, ordered by (slot, tx order, output order). Both asc and desc become a single page read (desc is a reverse range read — Blockfrost defines it as the exact reverse of asc).

• fjall (the supported index backend) gets a new stake-log keyspace with two entry shapes: a membership entry per pair (the write-path probe and the undo key) and an ordered entry (the page read). The write batch cannot read its own pending inserts, so the writer keeps a batch-local seen-set — one writer can span many blocks in WAL catch-up.
• memory implements the same semantics, so ToyDomain endpoint tests exercise the log. redb3 and noop answer None (redb3 is deprecated for index stores).
• Undo correctness: rollback does not re-run index_block, so compute_undo derives the appearances from the same stake_appearances_from_block function the apply path uses. A pair is removed only when the undone block is its stored first appearance; a block that merely repeated an address leaves the log untouched.
• Rollout: a ready marker gates reads. Genesis bootstrap sets it, so any store synced (or mithril-bootstrapped) from scratch serves the log; existing stores answer None and the endpoint transparently falls back to the archive scan from fix(minibf): fix ordering and 404 edge cases in account addresses #1221 until they resync. No doctor/backfill job — resync is the rollout path.
• Full address bytes are stored (not just the payment part) so pointer addresses round-trip. Estimated mainnet size: 5–15GB against a 340GB store.

Testing

• Backend conformance test in tests/index_roundtrip.rs runs against fjall and memory: marker gating, first-appearance dedup (including inside one multi-block writer), ordered paging from both ends, offset windows, stake isolation, and undo-of-first-appearance-only.
• Memory-store unit tests cover the same semantics at the store level.
• Endpoint equivalence test: two identical synthetic chains, one serving from the log and one from the archive-scan fallback, must return identical responses for every query shape. Verified non-vacuous by flipping the order flag and watching the order tests fail.
• Full workspace suite passes; clippy clean; nightly fmt applied.

Not yet done: an end-to-end run against a freshly synced network store (my local preview/mainnet stores predate the log, so they exercise only the fallback). Happy to re-sync preview and attach numbers if you want them before undrafting.

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[slowbackspace]

Applied into #1221 as commit 568d3ba. That PR now carries both the fixes and the stake address log.