fix: make Mermaid rendering responsive

[teng-lin]

What changed

• Make Mermaid rendering aware of the actual content width, including list and blockquote prefixes plus the surrounding frame.
• Keep the authored Mermaid layout when it fits.
• Retry oversized left-to-right flowcharts vertically before falling back.
• Retry oversized class diagrams horizontally when that produces a valid fit.
• Render class diagrams that still cannot fit as vertically stacked class cards followed by a compact relationship list.
• Fall back to syntax-highlighted Mermaid source when a valid two-dimensional render cannot fit safely, instead of wrapping rendered rows.
• Add demo.md with reproducible horizontal-flowchart and large-class-diagram examples.

Why

Mermaid output is a pre-rendered two-dimensional text canvas. Leaf previously passed oversized diagram rows through the generic code-block wrapper. Wrapping those rows preserved the characters but broke their spatial relationships, splitting box borders, labels, and connectors across unrelated terminal rows.

A width guard prevents that corruption, but source fallback alone means large class diagrams no longer render at ordinary terminal widths. Class graphs with many wide cards and fan-out relationships can remain hundreds of columns wide in every graphical orientation.

This change treats Mermaid layout as responsive content. It tries alternate graph orientations when useful and uses a vertical class representation when no graphical orientation can fit. Class names, stereotypes, members, and authored relationships remain visible and searchable while the output grows downward instead of beyond the viewport.

User impact

• Narrow terminals no longer show mangled Mermaid boxes and arrows.
• Wide terminals continue to receive the normal graphical rendering.
• Large class diagrams remain rendered and readable at common terminal widths rather than appearing as raw Mermaid source.
• Invalid or unsupported diagrams retain the existing source fallback.

Validation

• cargo fmt --all -- --check
• cargo test — 416 passed
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --release
• Manually rendered demo.md at 80 columns
• Manually rendered a 196 KB API design document containing the reported large class diagram at 80, 120, 240, and 320 columns

[RivoLink]

Hi,
Sorry for the delay, and thank you for your interest in the project and for this PR.

I have a few requests:

• Rebase from main and fix the CI check failure by adding : to the commit message: fix: responsive Mermaid layout
• Maybe remove the demo.md file from the commit and attach it to this PR instead.
• GPG signing is required on the main branch, so if you can sign it, great. If not, no worrie, I'll handle it while preserving you as the author.

GPG signing docs : https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits

Thanks!

[RivoLink]

Maybe remove this file from the commit and attach it to the PR