script context: add Observing purpose for CIP-0112

[wolf31o2]

Summary

CIP-0112 introduces a new Plutus script purpose, Observing, that runs a script as an observer (no direct redeemer action like spend/mint/etc.). plutigo's script context / script purpose model needs to represent this new case so CIP-0112 scripts can be evaluated once the CIP activates.

What Needs to Happen

1. Extend the ScriptPurpose / ScriptInfo surface to include an Observing case.
2. Ensure the ToData encoding matches the CBOR constructor tag defined by CIP-0112 for the target Plutus version (V4+ gated).
3. Accept Observing in the script-context builder path so downstream consumers can construct contexts for observe-purpose scripts.
4. Add unit tests covering encode/decode of the new purpose.

Gating

Only emit / accept Observing for the Plutus language version that activates CIP-0112 (expected PlutusV4 under Dijkstra). Earlier versions must keep rejecting it.

Upstream tracking

• gouroboros: ledger: implement CIP-0112 Observe script purpose gouroboros#1700

References

• CIP-0112