docs: document remaining duplication in CtxUTxO FromJSON instance

Adds inline documentation noting that the CtxUTxO instance still contains
~60 lines of duplicated code in the Babbage/Conway/Dijkstra cases that
could potentially be refactored using a similar approach to the CtxTx
instance refactoring.

The comment includes:
- NOTE: Identifies the specific lines containing duplication
- POTENTIAL REFACTORING: Suggests how it could be addressed (similar to
  parseInlineDatum helper in CtxTx)
- BLOCKER: Documents the key difference that must be preserved - CtxUTxO's
  alonzoTxOutParser doesn't handle supplemental datums, unlike CtxTx

This serves as documentation for future maintainers who may want to
complete the refactoring, while explaining why it wasn't done in this PR.

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: newhoggy

cardano-api/src/Cardano/Api/Tx/Internal/Output.hs

@@ -611,6 +611,19 @@ instance IsShelleyBasedEra era => FromJSON (TxOut CtxTx era) where
             <*> return ReferenceScriptNone
         (Just _dVal, Nothing) -> fail "Only datum JSON was found, this should not be possible."
 
+-- NOTE: The CtxUTxO instance below still contains significant duplication in the
+-- Babbage/Conway/Dijkstra cases (lines ~592-666). Each case has nearly identical
+-- inline datum parsing logic that could be extracted into a helper similar to the
+-- parseInlineDatum function in the CtxTx instance above.
+--
+-- POTENTIAL REFACTORING: The inline datum parsing at lines 596-611 (Babbage),
+-- 621-636 (Conway), and 646-661 (Dijkstra) could be unified using a helper that
+-- takes a BabbageEraOnwards witness, similar to how parseInlineDatum works in CtxTx.
+-- This would eliminate ~60 more lines of duplication.
+--
+-- BLOCKER: The CtxUTxO alonzoTxOutParser differs from CtxTx - it doesn't handle
+-- supplemental datums (only datum hash, no datum value). This difference would need
+-- to be carefully preserved in any refactoring.
 instance IsShelleyBasedEra era => FromJSON (TxOut CtxUTxO era) where
   parseJSON = withObject "TxOut" $ \o -> do
     case shelleyBasedEra :: ShelleyBasedEra era of