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

justfile

@@ -80,9 +80,10 @@ mutants-import:
 mutants-config:
     cargo mutants -p ledger-config --test-package ledger-config -j 4 --timeout 120 -o mutants-config
 
-# Mutation testing of the MCP server (every mutant runs the stdio suites).
+# Mutation testing of the MCP server (every mutant runs the stdio suites;
+# fewer random cases each, as for the store).
 mutants-mcp:
-    cargo mutants -p ledger-mcp --test-package ledger-mcp -j 4 --timeout 600 -o mutants-mcp
+    MCP_CASES=16 cargo mutants -p ledger-mcp --test-package ledger-mcp -j 4 --timeout 600 -o mutants-mcp
 
 # Mutation testing of the viewer's logic (formatting, file names, history,
 # search, ticks, the view model, the attention list; not the painting).