log: demote the four lines that were 90% of debug.log behind /debugrelay

A 13-hour production capture was ~71% "sending: tx", ~16% "Nostr:
discovered .btf peer", ~5% "Relaying wtx", and ~43% of raw lines were
CBlock/CTransaction dumps -- all already under -debug, which operators
run permanently, so -debug was useless as a filter.

The category mechanism gains a verbose tier: "relay" and
"nostrdiscovery" are accepted only with the new /debugrelay flag (which
implies /debug); everything else keeps its current gate. Demoted, with
message text unchanged for anyone grepping it:

  net.h     "sending: tx (N bytes)" (BeginMessage/EndMessage/
            AbortMessage all pick the category from the command, so no
            orphaned "(N bytes)" fragments leak into a plain -debug log)
  main.cpp  "Relaying wtx", "received block:" + CBlock::print, and the
            BitcoinMiner block dump
  nostr.cpp "Nostr: discovered .btf peer" -> "nostrdiscovery"

Pool/worker/payout, [payout], BLOCK ORPHANED/ACCEPTED and share flow
lines are untouched. Because LogPrint dedup keys on category, demoted
lines also keep their burst collapse under /debugrelay.

Closes #35

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