Rework roadmap: add Phase 2.5 adoption fixes, Phase 6 signing layer, expand Phase 4

- Phase 2.5 (Integration-friction fixes): multi-value encoding, request
  batching, signed errors/proof of absence, MCP server mode — small,
  focused additions that materially reduce integration cost.
- Phase 4 (Proof and confidentiality modes) now surfaces the delivered
  FHE plugin and encrypted-channel plugin alongside Reclaim TLS proofs,
  and promotes deterministic replay to name SP1/RISC Zero explicitly.
- Phase 5 trimmed — visual config builder dropped, operator dashboard
  and endpoint directory retained.
- Phase 6 (Signing layer for existing APIs) — framework middleware,
  API gateway plugins, reverse-proxy sidecar.
- Future section trimmed: 'TLS proofs at scale' dropped (Reclaim ships);
  Solana generalised to 'Chain ports'.
- x402 description in Phase 2 updated to reflect the payer-signed
  authorisation binding shipped in the recent security pass.

Also deletes IDEAS.md — its contents are either delivered, incorporated
into the roadmap, or no longer relevant to the v2 direction.

Author: andreogle