feat: say why a mermaid diagram was not rendered

[simono]

What changed

When no layout of a Mermaid diagram fits, leaf falls back to its source (#288). The block header now says why and how wide the terminal needs to be:

┌─ mermaid · not rendered, needs 256 columns, has 120 ─┐

• mermaid::render returns Result<String, Fallback>; Fallback::TooWide carries the width of the narrowest layout it tried.
• The header reports that in terminal columns, including the frame and any list or blockquote prefix, so resizing to that width renders the diagram.
• On narrow terminals the label drops detail until it fits instead of wrapping.
• Unsupported or invalid diagrams keep the plain mermaid header.

Why

The fallback is silent, so a diagram that is only too wide looks like unsupported Mermaid. I hit this with a flowchart and a state diagram that need 256 and 212 columns.

How it was tested

• New tests in markdown_embedded.rs: the reported width is exact (one column less falls back, the reported width renders), the header fits at widths 20 to 60, and unsupported diagrams get no note.
• cargo test, cargo clippy --all-targets --all-features -- -D warnings, cargo build --release.
• Manually with wide flowchart and state diagrams, resizing the terminal across the threshold.