Cardano Updates

A typed-nil pointer (e.g. (*common.InfoGovAction)(nil)) matched its
concrete case in conwayGovActionType and was accepted, wrapping a nil
action that encodes to CBOR null or panics in ToPlutusData. Check for an
untyped nil interface and a typed-nil pointer before the type switch so
all nil shapes are rejected at construction.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Signed-off-by: Chris Guiney <[email protected]>

Signed-off-by: Tripura Repalle <[email protected]>

Signed-off-by: Akhil Repala <[email protected]>

The new Conway governance constructors accepted inputs that produced
invalid or panic-prone objects. Tighten validation so construction
fails fast instead of deferring the failure to encode/PlutusData.

- NewConwayGovAction now returns an error for a nil or unsupported
  governance action instead of silently coercing it to discriminant 0
  (which equals ParameterChange); conwayGovActionType returns (uint, error)
- NewUpdateCommitteeGovAction rejects a zero-value cbor.Rat quorum, which
  has a nil inner *big.Rat and panics when CBOR encoded
- NewTreasuryWithdrawalGovAction and NewUpdateCommitteeGovAction reject
  nil map keys, which panic when the action is rendered to PlutusData
- add negative tests covering each rejected case

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Signed-off-by: Chris Guiney <[email protected]>

Building a proposal procedure or governance action previously required
populating concrete structs by hand, including the Type discriminant
field. Add constructors that auto-set the discriminant and validate
required fields.

- common/gov.go: NewGovAnchor, NewGovActionId, and constructors for the
  six common governance actions (hard fork, treasury withdrawal, no
  confidence, update committee, new constitution, info)
- conway/gov.go: NewConwayParameterChangeGovAction, NewConwayGovAction
  (wraps an action and sets the discriminant via a type switch), and
  NewConwayProposalProcedure
- validation covers 28-byte policy/script hashes, non-empty treasury
  withdrawals, and 32-byte anchor data hash / gov action tx id
- additive only; existing exported structs and encoding behavior are
  unchanged

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Signed-off-by: Chris Guiney <[email protected]>

Bumps [vitest](https://github.com/vitest-dev/vitest/tree/HEAD/packages/vitest) from 3.2.4 to 3.2.6.
- [Release notes](https://github.com/vitest-dev/vitest/releases)
- [Changelog](https://github.com/vitest-dev/vitest/blob/main/docs/releases.md)
- [Commits](https://github.com/vitest-dev/vitest/commits/v3.2.6/packages/vitest)

---
updated-dependencies:
- dependency-name: vitest
  dependency-version: 3.2.6
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Signed-off-by: Akhil Repala <[email protected]>

Signed-off-by: Akhil Repala <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Doc Holiday <[email protected]>

Signed-off-by: Akhil Repala <[email protected]>

Signed-off-by: Akhil Repala <[email protected]>

New backend