Reference stub peer + conformance kit — decouple #13 from TerraYield

[mortonanalytics]

TerraYield need (breaks a circular dependency). #13 (E2E: TerraYield peer renders in shell) is gated on TerraYield building its surfaces, while TerraYield's epic (morton-analytics-llc/eo_ag#22) justifies its surfaces by IONe rendering them. Neither side can prove anything first. Break the cycle inside IONe:

Acceptance

• A minimal stub peer (fixture app) implements the six-surface contract from md/design/app-integration-playbook.md: MCP server, view-hint metadata, whoami, context slice; canned map/chart/table/document refs.
• IONe's shell renders the stub peer's refs end-to-end (this re-scopes E2E test: TerraYield peer renders map/chart/table in workspace shell (AC-6 / OQ-2) #13 to run against the stub; TerraYield later becomes a second, real peer).
• A conformance test kit (script or test suite) that any candidate peer can run standalone against its own endpoint — pass/fail per surface — so TerraYield can validate without a running IONe.

When this closes, TerraYield builds against the kit, not against hope.

[mortonanalytics]

Landed in #23, merged as d38683a. (The PR body's Closes #12, #13, … list only auto-linked #12 — closing the rest manually.)