Render: sanitize lets Mermaid directives and angle brackets through from role text

[jbeda]

Found by the adversarial fixture run during review of #28. Pre-existing on main, not
a regression from that PR — filing separately so it doesn't creep into its scope.

sanitize in internal/render/mermaid/mermaid.go neutralizes backticks, quotes, square
brackets and newlines, but passes <, > and %% through untouched.

1. A role can restyle the entire diagram

Mermaid directives use %%{...}%%. A role containing one is emitted verbatim into the
generated diagram source and interpreted as a directive rather than as label text:

relationships:
  - entity: Policy
    cardinality: "1:n"
    role: "%%{init:{'theme':'dark'}}%%"

The rendered diagram comes out restyled. A directive is not limited to theming, so the
blast radius is whatever the consuming Mermaid build honours in a directive block.

This matters because *.modelith.yaml files are checked into repositories and rendered
into Markdown that gets committed and published — the input is not always written by the
person reading the output.

2. Angle brackets silently vanish

role: "<angle>"

renders as an empty-looking label — Mermaid's label handling treats it as markup. A model
author using <...> as a placeholder convention loses the text with no diagnostic.

Repro

Fixture and full evidence matrix from the #28 review round:

• .scratch/reviews/pr28-fixtures/f16-html-inject.modelith.yaml
• .scratch/reviews/pr28-adversarial.md

modelith render f16-html-inject.modelith.yaml
npx -y @mermaid-js/mermaid-cli@11 -i out.mmd -o out.png -b white

Confirmed identical on main and on the #28 branch.

Suggested fix

Extend sanitize to neutralize %% and to escape or replace < / >. Worth auditing at
the same time whether any other Mermaid metacharacter reaches a label unescaped, and
whether the Markdown renderer has the same class of gap — sanitize only guards the
Mermaid path.

Add fixtures for these to the renderer tests so the guarantee is pinned rather than
assumed.