docs(onchain-zaps): record the hand-rolled Bitcoin consensus code decision

Decision: keep the hand-rolled psbt/ + taproot/ consensus layer rather than
adopting fr.acinq.bitcoin-kmp. Rationale: a deliberately small single-key-path
P2TR subset, pinned to authoritative BIP-341/350 test vectors at every layer
(sighash, tweak, witness signature bytes, addresses, tx serialization), and
consistent with the project's minimal-dependency stance.

Recorded in amethyst/plans/2026-05-14-onchain-zaps.md with the consequence
spelled out (we own correctness; revisit if scope expands past single-key-path
P2TR). The psbt/ and taproot/ packages now carry a pointer back to that
decision so a future reader doesn't reflexively swap in a library.

Doc-comment + plan-doc only; no logic change.

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nipBCOnchainZaps/psbt/Psbt.kt

@@ -97,6 +97,12 @@ class PsbtMap(
  * finalize the single-key-path P2TR spends NIP-BC needs. Unknown records are
  * preserved verbatim so the container round-trips even when fields aren't
  * modeled.
+ *
+ * This `psbt/` package is intentionally hand-rolled rather than delegated to a
+ * Bitcoin library. That is a recorded architecture decision — see
+ * `amethyst/plans/2026-05-14-onchain-zaps.md` ("Architecture decision:
+ * hand-rolled Bitcoin consensus code"). It holds only while the scope stays at
+ * single-key-path P2TR; expanding past that should revisit the decision.
  */
 class Psbt(
     val global: PsbtMap,

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nipBCOnchainZaps/taproot/TaprootAddress.kt

@@ -37,6 +37,11 @@ import com.vitorpamplona.quartz.utils.sha256.sha256
  *
  * where `bytes(P)` is the 32-byte x-only Nostr pubkey and `hashTapTweak` is
  * the BIP-340 tagged hash with tag `"TapTweak"`.
+ *
+ * This `taproot/` package is intentionally hand-rolled rather than delegated to
+ * a Bitcoin library — a recorded architecture decision, see
+ * `amethyst/plans/2026-05-14-onchain-zaps.md` ("Architecture decision:
+ * hand-rolled Bitcoin consensus code").
  */
 object TaprootAddress {
     private const val TAP_TWEAK_TAG = "TapTweak"