mcp: HTTP concurrency test; state-machine suite tolerates a contended disk

tests/http_concurrency.rs: six HTTP clients at once against one store
(main.rs serve_http shares it): every bare entry refused at threshold
0.00, every receipted one posted, each contested draft posted exactly
once, books verify and balance, a restart replays the same head.

spec_mcp.rs: the response timeout is 180 s (a mutation run's jobs
contend for a synced disk; the baseline timed out at 30 s), and
MCP_CASES sets the state machine's cases (just mutants-mcp uses 16).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-mcp/Cargo.toml

@@ -34,5 +34,7 @@ toml = "1"
 workspace = true
 
 [dev-dependencies]
+tokio = { version = "1", features = ["time", "macros", "rt-multi-thread"] }
+rmcp = { version = "3", features = ["client", "transport-streamable-http-client-reqwest"] }
 jsonschema = { version = "0.58", default-features = false }
 proptest = "1"