Unify StaticHtmlEmitter single-page and site emission lifecycles

[chubes4]

Parent: #1076

Problem

StaticHtmlEmitter::emit() and emitSite() independently reset mutable state, normalize assets, extract design-system data, emit nodes, deduplicate CSS classes, resolve fonts, assemble files, and build diagnostics. Tests explicitly exercise fixes in both paths, demonstrating the maintenance burden.

Scope

• Introduce one internal emission lifecycle used by both public methods.
• Keep emit() and emitSite() as stable public entrypoints.
• Represent single-page emission as input to the shared page/root emission implementation.
• Remove duplicated initialization and finalization code once behavior is shared.

Acceptance criteria

• composer test passes.
• Existing single-page and multi-page HTML/CSS/assets/report contracts remain unchanged.
• Inline CSS defaults remain unchanged for each public entrypoint.
• Responsive variants, template aliases, link resolution, font handling, and diagnostic output retain current behavior.
• The parity assertions around both paths no longer require distinct implementation branches.

AI assistance

OpenAI gpt-5.6-sol via OpenCode identified the duplicated lifecycle and drafted this issue. Chris Huber is responsible for the scope and final changes.