Track mainnet signature / DSA support across schemes

## Background

Mainnet signing support is broader than the Falcon workstream alone. We need one release-level parent for cross-scheme concerns such as:

- scheme-specific implementation tracks like Falcon and secp256r1
- public-key commitment semantics across signature schemes
- signature encodings and interoperability concerns

## Scope

- Track the mainnet signature / DSA work across schemes
- Keep scheme-specific umbrellas or child issues nested below this parent where appropriate
- Separate Falcon-specific work from cross-scheme concerns
- Make it clear which signing concerns are mainnet-facing versus deferred