[Contracts] NftError and GovernanceError variants and event topic symbols are undocumented

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

remittance_nft/src/lib.rs:7-29 (NftError, 19 variants) and multisig_governance/src/lib.rs:35-57 (GovernanceError, 4001-4020) have no per-variant rustdoc explaining when each is returned. Additionally the emitted event topic symbols (Mint, ScoreUpd, NftBurned, MntAuth, UriUpd, HashUpd, AdmRemint, GovProp, GovAppr, GovFin, GovExp, etc.) are not documented anywhere (README.md has no event schema section) for indexer integrators.

Acceptance criteria

• Add a rustdoc line to every NftError and GovernanceError variant describing its trigger condition
• Document each emitted event's topic symbol and payload (struct or tuple) in rustdoc or README so indexers have a reference
• Clarify the BelowMinimum vs InvalidRepaymentAmount distinction in update_score docs

Files to touch

• contracts/remittance_nft/src/lib.rs
• contracts/multisig_governance/src/lib.rs

Out of scope

• Renaming error variants or event symbols
• Changing event payloads