Misc 20260904 fixes

[luke-jr]

No description provided.

[jasonsopko]

Tested ACK cd91837

Ubuntu 24.04, gcc 13.3, clang 18.1, on master b9ea7dc. gcc and clang with -Wall -Werror, ENABLE_API=OFF, DATUM_API_FOR_UMBREL and ASan all build clean and pass --test. The UBSan run stops at the pre-existing misaligned store in datum_protocol_tests.c:133; with #5 cherry-picked on top it passes.

Notes from reading the three commits:

• The leak fix is latent today: the only caller (datum_protocol.c:1355) passes must_free=false. Correct either way.
• outval > s->coinbase_value - tally is safe because tally only grows by values that already passed the check, so it never exceeds coinbase_value. The new test covers the overflow case.
• The vardiff move fixes a real first-notify problem. The client slot is zeroed on connect, so stratum_update_vardiff() inside the first send_mining_notify() saw share_snap_tsms == 0, read that as more than 60 s without shares, and halved current_diff before the forced_high_min_diff/vardiff_min clamps. Starting the snapshot before the notify makes that pass a no-op.