Never swap the tip's hash sideways, and never split it from its height (B-64)

_apply_header's linkage check only fires on a single-block advance, so a header
at the height we already held one for was applied on nothing but its own
self-consistency — and that check, as header_meets_its_own_target's own docstring
says, a server can satisfy with a self-declared easy target. So the one value the
draw is seeded from could be replaced under us at the current height, by a reorg
at the tip or by a single server disagreeing with the rest, with no check able to
speak to it. Separately, a header carrying no hex set tip_header_hex back to None
while advancing tip_height, leaving the two describing different blocks — the
exact pairing that function exists to keep.

Both are now refused without ending the session, unlike the fabrication cases
above them: neither is evidence of a hostile server, and rotating away would cost
us the one connection that also credits deposits and broadcasts transactions.

- A same-height header is ignored (logged when it actually differs). The hash
  committed to for a height is not swapped under us; if ours turns out to be the
  orphan, corroborate_header already refuses to seed a draw from it and the draw
  waits for a further block.
- A hex-less header is ignored outright: nothing to validate, nothing to draw
  from. A server that only ever pushed heights now freezes the draw — visibly,
  via B-36's draw_stalled — instead of costing us the connection.

Because ignoring is not fatal, _run_once additionally refuses to publish the
client when the initial header leaves the tip still unknown, so this cannot
reopen B-63's window from the other side.

The two _run_once tests are bounded with asyncio.wait_for: without their guard
that call waits on session tasks nothing ends, and a regression must fail rather
than hang the suite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -139,7 +139,7 @@ One connection serves everything — deposit credits, broadcasts, confirmations,
 - **Rotation.** `ELECTRUM_HOST`/`PORT` is primary, `ELECTRUM_FALLBACK_SERVERS` a comma-separated `host:port[:notls]` list (`client.py:parse_endpoints` rejects malformed entries at startup, not during the outage when the fallback is needed). After any failed or dropped session the next server is tried immediately; the backoff (1s doubling to 30s) only kicks in once every server has had a turn.
 - **Bounded requests** (`_REQUEST_TIMEOUT_SECONDS` = 15s); a timeout tears the connection down. Unbounded waits used to hang `POST /bets` *while holding the per-user lock*, and could stall the confirmation poller permanently.
 - **The drop is observable**: `client.wait_closed()` resolves when the read loop dies, and `_run_once` races it against the notification consumers and a 60s `server.ping`. Without it the listener sat on queues nobody would ever fill while `listener.client` still looked alive.
-- **Headers are validated, not trusted** (`_apply_header`): the tip never regresses, a header must meet the difficulty target it claims, and a single-block advance must chain from the current tip's hash. Failure raises `HeaderValidationError`, which ends the session like a dropped connection and rotates away — that header is the draw's only entropy, so a fabricated one picks the winner.
+- **Headers are validated, not trusted** (`_apply_header`): the tip never regresses, a header must meet the difficulty target it claims, and a single-block advance must chain from the current tip's hash. Failure raises `HeaderValidationError`, which ends the session like a dropped connection and rotates away — that header is the draw's only entropy, so a fabricated one picks the winner. Two more headers are refused *without* ending the session, since neither implies a hostile server (B-64): one at a height we already hold a header for (a reorg at the tip, or one server disagreeing — the hash committed to for a height is never swapped under us, and `corroborate_header` is what catches us holding an orphan), and one carrying no `hex` at all (nothing to validate or draw from, and applying the height alone would break the `tip_height`/`tip_header_hex` pairing). `_run_once` separately refuses to publish the client while *no* tip is known, so the ignore-don't-kill choice can't reopen B-63.
 - **A quorum corroborates every money-moving decision** (`_corroborate_majority`, 10s per server, asking only the *other* endpoints — never the active one, which is what a MITM controls): `corroborate_header` before a block seeds the draw (B-28), `corroborate_utxo_spent` before a UTXO missing from one `listunspent` is written off as externally spent (B-29), and `corroborate_utxo_credit` before a new outpoint credits a balance — same outpoint, same amount, confirmed (B-59). Balances move in both directions, so both directions need the same quorum. No fallbacks configured → returns True (the accepted risk of an empty `ELECTRUM_FALLBACK_SERVERS`); nobody answers → returns **False**, since an unreachable network proves nothing. A failed credit corroboration only *delays*: `find_new_credit_candidates` re-offers the outpoint on the next refresh or `DepositReconciler` sweep.
 
 On reconnect `_subscribe_all_users` runs as its own task with bounded concurrency (`_RESUBSCRIBE_CONCURRENCY` = 20) rather than inline and serially — otherwise a large user base froze `tip_height`, and with it an in-flight draw, for the whole sweep (B-31); one user's failure is logged and skipped. `address_for_new_user` (called right after registration) is best-effort by design: on failure that address stays unsubscribed until the next reconnect or `DepositReconciler` sweep.