chore: add ROADMAP.md tracking gaps from tier 1 (#713)

ROADMAP.md

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+# RMCP Roadmap
+
+This roadmap tracks the path to SEP-1730 Tier 1 for the Rust MCP SDK.
+
+Server conformance: 86.7% (26/30) · Client conformance: 85.0% (18/24) · Spec tracking gap: 6 days
+
+---
+
+## Tier 2 → Tier 1
+
+### Conformance
+
+#### Server (86.7% → 100%)
+
+- [ ] Fix `server-prompts-get-with-args` — prompt argument handling returns incorrect result
+- [ ] Fix `server-prompts-get-embedded-resource` — embedded resource content in prompt responses
+- [ ] Fix `server-elicitation-sep1330-enums` — enum inference handling per SEP-1330
+- [ ] Fix `server-dns-rebinding-protection` — validate `Host` / `Origin` headers on Streamable HTTP transport
+
+#### Client (85.0% → 100%)
+
+- [ ] Fix `auth/scope-step-up` (2025-11-25) — handle 403 `insufficient_scope` and re-authorize with upgraded scopes
+- [ ] Fix `auth/metadata-var3` (2025-11-25) — AS metadata discovery variant 3
+- [ ] Fix `auth/2025-03-26-oauth-endpoint-fallback` (2025-03-26) — legacy OAuth endpoint fallback for pre-2025-06-18 servers
+
+### Governance & Policy
+
+- [ ] Create `VERSIONING.md` — document semver scheme, what constitutes a breaking change, and how breaking changes are communicated
+
+### Documentation (26/48 → 48/48 features with prose + examples)
+
+#### Undocumented features (14)
+
+- [ ] Tools — image results
+- [ ] Tools — audio results
+- [ ] Tools — embedded resources
+- [ ] Prompts — embedded resources
+- [ ] Prompts — image content
+- [ ] Elicitation — URL mode
+- [ ] Elicitation — default values
+- [ ] Elicitation — complete notification
+- [ ] Ping
+- [ ] SSE transport — legacy (client)
+- [ ] SSE transport — legacy (server)
+- [ ] Pagination
+- [ ] Protocol version negotiation
+- [ ] JSON Schema 2020-12 support *(upgrade from partial)*
+
+#### Partially documented features (7)
+
+- [ ] Tools — error handling *(add dedicated prose + example)*
+- [ ] Resources — reading binary *(add dedicated example)*
+- [ ] Elicitation — form mode *(add prose docs, not just example README)*
+- [ ] Elicitation — schema validation *(add prose docs)*
+- [ ] Elicitation — enum values *(add prose docs)*
+- [ ] Capability negotiation *(add dedicated prose explaining the builder API)*
+- [ ] Protocol version negotiation *(document version negotiation behavior)*
+
+---
+
+## Informational (not scored)
+
+These draft/extension scenarios are tracked but do not block tier advancement:
+
+- [ ] `auth/resource-mismatch` (draft)
+- [ ] `auth/cross-app-access-complete-flow` (extension)
+- [ ] `auth/client-credentials-jwt` (extension)