Add hardened seconv MCP stdio server

[Blackspirits]

Purpose

Current-base audited port of upstream PR SubtitleEdit#14129 (seconv mcp), stacked on draft #109.

The original PR was based on adbc446a... and is now more than 1,000 upstream commits behind. Its MCP surface still maps cleanly onto current seconv, but a literal port would miss important long-lived-server concerns and would overstate parity with the much-expanded current CLI.

What this PR adds

• seconv mcp stdio server using ModelContextProtocol 2.2.0;
• current Microsoft.Extensions.Hosting 10.0.12;
• tools:
  • list_formats
  • subtitle_info
  • read_subtitle
  • lint_subtitle
  • convert_subtitle
  • list_fix_common_errors_rules
  • list_remove_formatting_rules
• stdout reserved for MCP JSON-RPC; incidental console/log output goes to stderr;
• CLI help, --help-json, README and current command-line reference updated.

Hardening vs. SubtitleEdit#14129

• builds on Make seconv conversion cancellation-aware and serialize global state #109 so conversion has a real cooperative cancellation contract and process-global conversion state is serialized;
• MCP CancellationToken is accepted explicitly and is therefore bound by the SDK without appearing in tool input schemas;
• all MCP calls are serialized inside the server while libse/seconv still depend on process-wide settings;
• caller cancellation is rethrown, not converted into an ordinary tool error;
• completed write operations are not retroactively reported as cancelled;
• read/list tools explicitly advertise ReadOnly=true, Destructive=false, Idempotent=true, OpenWorld=false;
• convert_subtitle explicitly advertises ReadOnly=false, Destructive=true, Idempotent=false, OpenWorld=false;
• known client validation errors remain actionable; unexpected internal exceptions are logged to stderr and return a generic tool error instead of leaking raw exception details/paths;
• missing-file errors expose only the file name, not the full local directory;
• documentation no longer falsely claims every subcommand accepts --json;
• the MCP conversion surface is documented as a focused subset of the current CLI rather than claiming complete option parity.

Filesystem boundary

The server operates on explicit local filesystem paths and does not add a second sandbox. Access remains limited by the OS permissions of the seconv process and by the MCP client's own approval/sandbox policy. MCP roots were considered, but roots are deprecated in the 2026-07-28 protocol revision, so this port does not build a security contract on that retiring mechanism.

Tests

The original functional coverage is retained and extended to verify:

• real stdio server startup and tool calls;
• tool annotations returned by tools/list;
• CancellationToken is absent from generated input schemas;
• pre-cancelled conversion throws and writes nothing;
• missing-file errors do not expose the test directory;
• conversion/offset and rule-selection behavior still use the shared seconv core.

Stack / validation

• base: draft Make seconv conversion cancellation-aware and serialize global state #109, HEAD f83e30793665214f054f411feb0c2254b8b68d8a
• this branch is exactly one commit ahead of that base
• HEAD: 946a7f00f19a91fcd4303a224ff169d4b771b75e
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• SeConvTests: 498 passed, 2 skipped, 0 failed (500 total)
• LibUiLogicTests: 873 passed, 0 skipped, 0 failed
• LibSETests: 2,008 passed, 0 skipped, 0 failed
• UITests: 5,151 passed, 9 skipped, 0 failed (5,160 total)
• retry step was not used
• validation run: #34795072886
• audit draft only; no merge/promotion intended

AI assistance: ChatGPT was used to audit and port the stale MCP proposal, reconcile it with current seconv behavior, and add cancellation, concurrency, metadata and error-boundary hardening.

[Blackspirits]

Independent adversarial re-check: this branch is exactly one MCP/docs/tests commit on the final #109 cancellation/concurrency base. The stdio transport uses Console.OpenStandardOutput directly, so redirecting incidental Console.Out writes to stderr does not corrupt JSON-RPC. Pre-dispatch bootstrap/HarfBuzz initialization does not write stdout. Client-visible tools/list annotations, cancellation-token schema exclusion, sanitized missing-file errors, cancellation-before-write and an end-to-end stdio tool call are covered by tests. Full CI #34795072886 passed restore, build and the complete suite on the first run; SeConvTests reported 498 passed, 2 skipped, 0 failed and UITests 5,151 passed, 9 skipped, 0 failed. Retry was not used. No blocker identified. Keep draft; no merge performed.