feat(dupes): add repo-wide structural duplicate clustering

[srobinson]

Summary

• Add Tier 1 fmm dupes repo-wide structural duplicate clustering with blocking, bounded overflow diagnostics, union-find clustering, deterministic stats, and JSON output.
• Reuse the existing similar candidate collector and structural scorer through shared core seams.
• Add CLI and MCP surfaces, generated tools metadata, command tree split, and focused coverage for blocking, clustering, CLI output, JSON stability, and MCP schema/tool behavior.

Test plan

• just check
• cargo test -q -p fmm-core dupes
• cargo test -q -p fmm --test cli_dupes
• cargo test -q -p fmm --test mcp_tools dupe_clusters
• cargo test -q -p fmm --test mcp_protocol mcp_protocol_tools_list
• cargo test -q -p fmm --test cli_flags tools_toml_cli_flags_are_exposed_by_clap_commands