docs: gnomon.md has 28 em dashes and 6 Title Case headers

[forkwright]

This is the most em-dash-heavy file in the repo. All 28 need conversion per WRITING.md ("No em dashes. Use commas, parentheses, colons, or separate sentences."). Examples:

• L5: "names work the same way — they don't describe" → colon
• L11: "engineered across centuries for a single purpose — distinguishing" → colon
• L15: "you're not saying 'thinking' — you're saying" → semicolon
• L21: "four syllables carrying an entire ontology — truth as" → colon
• Table cells (L84-116): em dashes in descriptions → parentheses

Title Case headers (6): Building Names, The Layer Test, The Gnomon Principle, When Naming, Anti-Patterns, and one more.