fix: show decoder context on dry-run decode fallback/failure

Summary

decodeCall in tx.ts (lines ~268-285) silently catches errors from both the primary and fallback decoders. When decoding fails, users see only (unable to decode) with no explanation of what went wrong.

Current Behavior

Call Data: 0x1f0003...
Decoded: (unable to decode)

Expected Behavior

Call Data: 0x1f0003...
Decoded: XcmPallet.reserve_transfer_assets(dest, beneficiary, assets, fee_asset_item)  (fallback decoder)

Or on complete failure:

Call Data: 0x1f0003...
Decoded: (unable to decode)
  Primary decoder: TypeNotFound: VersionedLocation not in registry
  Fallback decoder: IndexOutOfRange: pallet index 31 call index 8 not found

Proposed Changes

• src/commands/tx.ts:
  • Capture error message from primary decoder (currently catch {} on line ~277)
  • Annotate output when fallback decoder succeeds: (fallback decoder)
  • When both decoders fail, show both error messages to aid debugging
  • Use --verbose flag to control whether full error details are shown (default: show brief hint)

Related Issues

• #66 (dot decode command) — when decode is extracted to a shared module, this context logging carries over.

Priority

P2 — Small improvement to debuggability. Quick win (size S).

Start in src/commands/tx.ts at decodeCall, especially the primary and fallback decoder catches around lines 268-285. Trace the dry-run decode output and how --verbose is handled, then verify that fallback success identifies the fallback decoder and complete failure reports the relevant decoder context while the default remains brief.