System.Events decoding fails for V11/V12 metadata blocks

Summary

System.Events storage cannot be decoded for historical blocks that use V11/V12 metadata. This is a metadata version incompatibility issue, not a lazy migration issue (those are tracked in #85).

Affected Blocks

Errors Observed

CannotDecodeValue { ty: Vec<EventRecord<T::Event, reason: VariantNotFound(72) at .[2].event }
CannotDecodeValue { ty: Vec<EventRecord<T::Event, reason: CodecError("Not enough data to fill buffer") at .[1].event.[0].[1] }
CannotDecodeValue { ty: Vec<EventRecord<T::Event, reason: CodecError("Not enough data to fill buffer") at .[1].event.[0].[0].[0].[15] }

Root Cause (Investigation Needed)

The issue appears to be related to how event pallet indices are encoded in V11/V12 metadata vs V14:

• V11/V12: Events may use a different pallet index encoding scheme
• V14: The current decoder assumes V14-style pallet indices

When decoding V11/V12 events with V14 assumptions, the decoder either:

1. Encounters an unknown variant (VariantNotFound(72))
2. Runs out of data because it's reading the wrong structure

TODO

• Confirm root cause - is it pallet index encoding or something else?
• Check what Polkadot-JS does for these blocks - does it decode successfully?
• Determine what changes would be needed to support V11/V12 event decoding
• Decide if this is worth fixing or if those blocks should simply be excluded from testing

Related

• #85 tracks lazy migration issues (mixed data formats within same spec version)
• This issue is specifically about metadata version incompatibility

Start by reproducing the listed Kusama blocks, especially 2064962-2064965, 2201991-2201995, and 2671528, and inspect the reported decoding errors. Compare their V11/V12 event decoding with Polkadot-JS, then determine whether pallet-index encoding is the cause. Done means confirming the root cause and documenting the required support or an explicit decision to exclude these blocks.