Publish copyable filesystem chrome recipe

[lbliii]

Saga: #329 · Epic: #334

Application chrome stays recipe-first. App authors need a copyable filesystem recipe, not a mega-shell macro.

Goal

Publish guidance around _layout.html, _context.py, _meta.py, route tabs, shell actions, command triggers, page tools, and local fragment endpoints.

Tasks

• Document filesystem-mounted app pages as recommended adoption path.
• Teach shell, page-root, and inner-fragment HTMX target ownership with concrete examples.
• Link from navigation pattern docs and showcase shell recipes section.
• Cite existing browser fixtures as regression proof.

Acceptance

• App authors can copy recipe without application_chrome() macro.
• Docs contract tests pass for referenced block/fragment names.

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Plan: docs/plans/PLAN-application-chrome-system.md, docs/plans/PLAN-pre-1.0-productization-saga.md Epic 6.