Give labeled navigation sections one owner

[chubes4]

Summary

• move labeled non-nav navigation-section recognition from HtmlTransformer into NavigationPattern
• replace the flow converter callback with a typed PatternContext dependency while preserving its original precedence
• remove three navigation-only methods from the generic source classifier

This advances #242 by reducing decisions reachable through HtmlTransformer rather than adding another engine abstraction. HtmlTransformer loses 57 lines and one method, closure context fields drop from 264 to 263, and production code decreases by 10 lines.

Verification

• composer test
• 292 PHP parity fixtures passed
• 385 HTML fixtures: 0 serialized-block hash differences, 0 exceptions
• focused regressions cover labeled social-link precedence and exact classifier boundaries

AI assistance

OpenAI GPT-5.6 Sol via OpenCode mapped the conversion architecture, implemented the navigation ownership consolidation under Chris Huber's direction, performed dedicated precedence reviews, and ran the full verification suite.