docs: add comprehensive 8-stage roadmap for echidna-as-a-service

Roadmap articulates the full path from proof-solving library (Stage 1)
to public-facing proof-as-a-service platform (Stage 8):

Stage 1: Autonomous proof execution (current; blocks on container isolation)
Stage 2: Bot-driven repository automation (echidnabot discovers & solves proofs)
Stage 3: Estate-wide service discovery (any bot anywhere can reach echidna)
Stage 4: Versioned developer API (REST v1.0, gRPC v1.0, client SDKs)
Stage 6: Human-facing web UI (proof submission, real-time solving, history)
Stage 7: Public external API (rate-limited, SLA, enterprise tiers)
Stage 8: Cross-platform PWA + desktop (Tauri, iOS, Android)

Critical blockers documented:
- Container isolation (Stage 1 production gate)
- GNN model training (Stage 3 quality gate)
- Bot-modes wiring (Stage 2 gate)
- Chapel dispatch routing (Stage 3+ parallel search)
- HTTP-capability-gateway (Stage 7 security)
- Contractile verbs (3/6; standards compliance)

Infrastructure checklist per stage; decision log for reference.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: hyperpolymath