Say what bets and withdrawals actually exclude (B-70)

The flowchart's WITHDRAW node (E1) stated that a withdrawal cannot happen
together with a bet in progress. The code only serializes the two *builds*
through the per-user lock: a withdrawal is accepted while a bet is still
unconfirmed, as long as confirmed, unspent UTXOs cover it.

CLAUDE.md makes every node of the diagrams binding, so one of the two had
to move, and it is the diagram. The hazard the node was reaching for is
the two transactions picking the same UTXO, and that is already excluded
twice: app/tx/locks.py keeps the builds from overlapping, and select_utxos
skips anything already marked spent_txid. What the node forbade on top of
that is spending untouched, confirmed money — so implementing it as
written would freeze a user's whole balance for a block after every bet
and protect nothing. E1 now describes the real rule, and CLAUDE.md's
per-user-lock paragraph states it is the only exclusion between the two.

Regenerated the A4/A3 PDFs (gitignored, so not in this commit).

The regression test is behavioural, not a wording check: it funds a user
with two confirmed UTXOs, bets (taking the larger), and asserts the
withdrawal goes through on the other one with the bet still unconfirmed
and neither transaction spending the other's input. A second test keeps
the diagram from drifting back.

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

flowchart/platform-overview.mmd

@@ -36,7 +36,7 @@ flowchart LR
 
     subgraph WITHDRAW["FASE 5 - Prelievo"]
         direction TB
-        E1["L'utente richiede un prelievo:\nindirizzo esterno + importo\n(non puo' avvenire insieme\na una scommessa in corso)"] --> E2["Si prepara e firma la transazione:\ndal suo indirizzo verso\nl'indirizzo esterno indicato\n(con resto che torna a lui)"]
+        E1["L'utente richiede un prelievo:\nindirizzo esterno + importo\n(spende solo saldo confermato\ne non ancora impegnato: una scommessa\nin attesa di conferma non lo blocca,\nma le due operazioni non vengono\nmai preparate nello stesso momento)"] --> E2["Si prepara e firma la transazione:\ndal suo indirizzo verso\nl'indirizzo esterno indicato\n(con resto che torna a lui)"]
         E2 --> E3["Transazione inviata\nalla rete"]
         E3 --> E4{"Confermata?"}
         E4 -- "No, troppo tempo" --> E5["Si aumenta la commissione\ne si reinvia"]