Shell response/OOB target ownership doc

[lbliii]

Saga: #329 · Epic: #334

Shell response/OOB routing stays route-local and recipe-first until repeated target-branching boilerplate justifies helpers.

Goal

Document HTMX shell/page-root/inner-fragment target ownership with concrete target IDs and OOB swap examples.

Tasks

• Map response contracts: full shell vs page-root vs inner fragment.
• Document OOB regions for shell actions, toasts, and nav badge updates.
• HTMX examples with concrete #target IDs from showcase filesystem pages.
• Cite tests/test_shell_response_targets.py and filesystem chrome tests.

Acceptance

• App authors understand which response shape to return for each navigation action.
• No new HTMX protocol or shell helper APIs without proof analysis approval.

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Plan: docs/components/shell-tabs-contract.md, docs/reference-implementations/PROOF-ANALYSIS.md.