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>

BUGS.md

@@ -40,21 +40,6 @@ remains the last prerequisite for running unattended.
 
 ## Medium — correctness and robustness
 
-### B-64 — `_apply_header` accepts a same-height header without the chaining check
-
-`app/electrum/listener.py:271-296`.
-
-The chain check only runs for `height == self.tip_height + 1`. A header at exactly
-the current tip height replaces `tip_header_hex` after passing only the
-self-target check — which, as the docstring of `header_meets_its_own_target`
-already notes, a server can satisfy with a self-declared easy target. Separately,
-a header with no `hex` field sets `tip_header_hex = None`, discarding a tip we
-otherwise accepted.
-
-Fix: treat a same-height header as either ignorable or as a reorg signal rather
-than silently replacing the entropy source, and don't clear `tip_header_hex` on a
-hex-less notification.
-
 ### B-65 — `/rounds/current` counts unconfirmed participants; the draw and payout do not
 
 `app/api/routes/rounds.py:131-143` vs `app/rounds/scheduler.py:126`.