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>

app/electrum/client.py

@@ -287,6 +287,12 @@ class ElectrumClient:
     async def get_block_header(self, height: int) -> str:
         return await self.request("blockchain.block.header", [height])
 
+    async def get_block_headers(self, start_height: int, count: int) -> dict:
+        """`count` consecutive headers from `start_height` (B-74, ELC-12), as the server's
+        {"count", "hex", "max"}: `hex` is the 80-byte headers concatenated, and the server
+        may return fewer than asked (never more than its `max`)."""
+        return await self.request("blockchain.block.headers", [start_height, count])
+
     async def _read_loop(self) -> None:
         """Frame the stream into lines by hand rather than with StreamReader.readline:
         readline raises on a line over the limit, and that exception used to end this