Navigation: use content hash for sidebar tree IDs

[reakaleek]

What

• Give each sidebar navigation tree a unique nav-tree-* id based on its rendered content, not the navigation root id
• Build navigation HTML from a render model so templates no longer depend on domain navigation types
• Cache rendered navigation once per navigation root during a build

Why

• Different pages can show different sidebars but still share the same nav-tree-* id when docsets lack a project key in docset.yml
• htmx hx-preserve then keeps the wrong sidebar DOM and expand/collapse state when moving between those pages

Notes

• The content hash is 16 lowercase hex characters and only covers the preserved tree, not the dropdown or search
• Codex and assembler builds both use the same render and cache path

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