Require real proof of work in headers and link every advance to the tip (B-74, ELC-11, ELC-12, ELC-22, ELC-23)

A header only had to meet the target it declared for itself, so a fabricated one with
`bits = 0x207fffff` passed half the time, a jump of several blocks was accepted without
being linked to the tip (a fake header at a huge height locked `tip_height` and froze every
draw), and an ordinary one-block reorganisation closed the session on every server.

- draw.py: MAINNET_POW_LIMIT (00000000ffff...ffff); header_meets_its_own_target also
  refuses a claimed target easier than POW_LIMIT.
- ElectrumListener._apply_header is async and takes the active client. A single block must
  chain by prev_hash; a jump downloads the headers in between (blockchain.block.headers,
  at most 2016 per request, ElectrumClient.get_block_headers) and each must carry valid
  work and chain, otherwise HeaderValidationError ends the session.
- The listener keeps the last 10 headers (`_window`), built from the process's first header
  and its 9 predecessors (which must chain into it). A header that does not join the tip is
  walked back to its fork point inside the window and the server's branch is followed
  instead of ending the session; a fork older than the window, or a branch that does not
  end at the announced header, still ends it. The audit row and the pause for deep reorgs
  are B-85; a competing header at the tip's own height is still ignored until then.
- _header_at also serves older heights from the window.

Tests changed because the spec changed: the mainnet powLimit cannot be mined in a test, so
tests/unit/conftest.py swaps in a regtest limit at import time (the new tests put the real
one back); the _apply_header tests are async; the two _run_once tests serve the first
header's 9 predecessors; test_apply_header_skips_linkage_check_across_a_height_gap is
replaced by the jump tests, since that behaviour is exactly what the finding removed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/unit/conftest.py

@@ -23,3 +23,12 @@ def _throwaway_master_key(tmp_path, monkeypatch):
     hd.generate_master_key()
     yield
     hd._account_key = None
+
+
+# B-74: the real powLimit needs ~2^32 hashes per header, so tests mine headers at a trivial
+# target (`0x207fffff`) and this swaps in a limit that allows it. Done at import, not in a
+# fixture, because some test modules mine headers while they are being collected. The
+# tests that exercise the limit itself put `MAINNET_POW_LIMIT` back with monkeypatch.
+from app.rounds import draw as _draw  # noqa: E402
+
+_draw.POW_LIMIT = (1 << 256) - 1