Agent Plugin MCP Package Fixture

A clean external Agent Plugins 1.0 package (see ADR-007) declaring exactly one MCP server, no Agent Skill. Kept separate from tests/_fixtures/canonical/skill-plugin/ deliberately: extending that package in place would change which resource several existing tests' .first()/unqualified assumptions resolve to, since resources sort by identity string and "mcp.json" sorts before "skills/...".

mcp.json's command field is a placeholder (__UZE_MCP_FIXTURE_BINARY__) — tests that need to actually spawn the server rewrite it to the real, test-build-resolved path of the uze-mcp-conformance-fixture binary (env!("CARGO_BIN_EXE_ uze-mcp-conformance-fixture")) before installing this fixture into a store. Tests that only exercise resource discovery/composition can use the placeholder as-is.

The fixture server itself (tests/_fixtures/bin/mcp_conformance_fixture.rs) is a minimal, real stdio MCP server built with the official rmcp Rust SDK. Its one tool, uze_conformance, returns whatever UZE_MCP_CONFORMANCE_PROOF was set to at server-launch time — a value the test controls, not a hardcoded constant, and deliberately independent of the Agent Skills PROOF token so the two conformance suites stay separately verifiable. No network, database, external API, credentials, or LLM is involved.