Evaluate provider-optimized generation after the provider-first generator contract stabilizes

[cwinland]

Docs reference: docs/roadmap/generator-roadmap.md

This issue is a late evaluation track for provider-optimized generation once the provider-first generator contract is already stable.

Acceptance checklist

• The candidate provider-optimized generation paths are enumerated per provider rather than described abstractly.
• Supported versus unsupported provider-optimized candidates are explicit before any implementation begins.
• The issue is explicitly gated on the provider-first generator contract and implementation work being stable first.
• Any future implementation plan keeps provider-specific semantics explicit instead of implying broad shared parity.
• The evaluation records why a provider-optimized path is worth adding beyond the existing provider-first contract.

Non-goals

• this issue does not block the main provider-first generator story
• this issue does not reopen v4 helper-surface scope