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>

docs/sicurezza.md

@@ -66,10 +66,9 @@ ricompensa di quel blocco.
   confermato dal quorum ([ELC-15](connessione-electrum.md#5-il-quorum)).
   *Oggi* (B-73): il seme è l'hash del primo blocco visto dopo la chiusura,
   senza segreto né impegno, e l'estrazione è manipolabile da chi mina.
-- **Header falsi.** Oggi l'unico controllo di lavoro su un header è che il
-  suo hash rispetti il bersaglio che l'header stesso dichiara nel campo
-  `bits`: poiché è chi fabbrica l'header a scegliere `bits`, un header mai
-  minato con `bits = 0x207fffff` lo passa con probabilità 1/2. Dopo B-74 il
+- **Header falsi.** Il bersaglio che un header dichiara nel campo `bits` lo sceglie chi
+  fabbrica l'header (con `bits = 0x207fffff` un header mai minato passava con
+  probabilità 1/2): per questo dal B-74 il
   bersaglio non può essere più facile del powLimit di PLM, `0x1d00ffff`
   ([ELC-11](connessione-electrum.md#4-header-validati-non-creduti)), e ogni
   salto di più blocchi è verificato come catena. Anche al powLimit un header