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

This is a tracking issue for coordinating mainnet signature support across multiple cryptographic schemes. It does not specify concrete code changes. To understand the scope, look at existing signature scheme implementations in the codebase, such as Falcon and secp256r1. The work involves design decisions about public-key commitments, signature encodings, and interoperability.

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