fix(fsm): preserve RetiredCommittees across genesis export/import

[Alicepoltora]

Description

GenesisState serializes through a hand-written JSON codec (the genesisState struct in fsm/genesis.go). That struct omits RetiredCommittees, and because GenesisState defines its own MarshalJSON/UnmarshalJSON, the proto tag on the real field is never used.

The result is that retired committees do not survive a genesis round trip:

• ExportState correctly populates genesis.RetiredCommittees (fsm/genesis.go).
• The field is silently dropped on marshal, so it never reaches genesis.json.
• On the way back in, SetRetiredCommittees(genesis.RetiredCommittees) inside NewStateFromGenesis always receives an empty slice.

So after a state export -> genesis.json -> restart (the documented upgrade/fork path), every previously retired committee is live again: it re-qualifies in GetSubsidizedCommittees and therefore starts receiving freshly minted tokens from FundCommitteeRewardPools again, and HandleCertificateResults stops rejecting its certificates. RetireCommittee is documented as making a committee non-subsidized "for eternity", but that property currently does not survive an export/import.

This is not attacker-controlled - it affects operators and governance performing an export-import upgrade.

Related Issues

None that I could find - I searched open and closed issues and PRs for RetiredCommittees / genesis serialization and found nothing covering this. Happy to open an issue first if you prefer that flow for changes like this.

Changes Made

• Added the RetiredCommittees field to the genesisState JSON codec struct, with a comment explaining why it must be serialized.
• Wired it through GenesisState.MarshalJSON and GenesisState.UnmarshalJSON.
• Added TestGenesisJSONRoundTripPreservesRetiredCommittees, which marshals a GenesisState carrying retired committees and asserts they are still present after unmarshaling.

The change is intentionally limited to the serialization gap; no behavior outside the codec is touched.

Checklist

• I have tested the changes locally and verified they work as intended.
• I have appropriately titled my branch issue-#<issue-number>. (No issue exists for this; the branch is named after the fix. Happy to rename if you'd like an issue opened first.)
• I have run re-built the web-wallet and/or block explorer (if applicable). (Not applicable - Go only.)
• I have run npm run prettier to format the web-wallet and/or block explorer (if applicable). (Not applicable.)
• I have updated documentation (if applicable). (Not applicable.)
• I have included tests for the changes (if applicable).

Additional Notes

Verification performed on development:

• The new test fails before the fix (the marshaled JSON contains no retiredCommittees key) and passes after it.
• go test ./fsm/ passes in full.
• go test $(go list ./... | grep -v cmd/auto-update) - i.e. the CI test command - passes across the repository.
• go build ./... succeeds and gofmt reports no changes for both touched files.
• go vet ./fsm/ surfaces only pre-existing warnings (in account_test.go, older genesis_test.go table cases, and generated message.pb.go); nothing from this change.

Two small things I noticed while here but deliberately left out to keep this PR single-purpose:

1. CONTRIBUTING.md asks contributors to update .docs/CHANGELOG.md, but neither that file nor the .docs directory exists in the repository. Glad to add a CHANGELOG in a separate PR if you want one.
2. The same genesisState codec also drops the export-only Committees field. I left it alone since it is a different concern; happy to follow up if it is also unintended.