fix: MCP path tools resolve against served source root (path/root contract coherence, slice D)

[srobinson]

Summary

• Fixes Finding D from the live agent road test: path tools resolved against process CWD and rejected absolute paths emitted by md_search.
• Routes md_context, md_structure, md_links, and md_backlinks through resolveMcpDocumentPath -> resolveIndexedDocumentKey.
• Canonicalizes accepted paths to the same absolute, realpath DocumentKey emitted by md_search, including symlink aliases.
• Returns the clear typed Path not in indexed corpus signal for paths outside every served root.
• Establishes the round trip invariant in a shared heterogeneous multiple root acceptance suite.
• Conforms to the unanimous path/root contract coherence council consensus.
• L2 follows with the canonical path_filter matcher and will extend the shared suite.

Test plan

• Shared MCP path round trip acceptance suite passes: 4 tests.
• Focused MCP and canonical indexing suite passes: 62 tests.
• Typecheck passes.
• Production build passes.
• Complete test matrix passes: 1,715 passed, 11 intentionally skipped, 0 failed across 1,726 tests.