fix: stabilize mermaid subgraph title width

[starSumi]

Cherry-pick of the Mermaid title width ceil fix split out of #155.

Validation:

• cargo test --locked --bin ferrite test_subgraph_title_width_expansion -- --nocapture
• git diff --check

This intentionally excludes the video autolink change to keep the PR aligned with the current {{video URL}} direction.

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[starSumi]

CI follow-up: Linux and Nix are green on #159. Windows still fails only on the existing baseline test vcs::git::tests::test_git_service_untracked_file (Clean vs Untracked), so this PR does not appear to be the cause of the Windows failure.