[seanwevans]

Motivation

• Reduce client-side complexity by moving HTML fragment rendering and escaping into the database so HTMX can perform native fragment swaps.
• Ensure untrusted command/output values are safely escaped server-side to eliminate client-side JSON parsing and DOM construction for RPC responses.
• Provide a clear PostgREST-compatible HTML contract so the frontend can request text/html fragments directly.

Description

• Add sql/html_fragments.sql which declares a "text/html" domain, adds pg_shell_html_escape, and provides latest_output_html and submit_command_html RPCs that return escaped HTML fragments.
• Register the new SQL file in sql/install.sql so the HTML RPCs install with the rest of the schema.
• Update the static UI in html/index.html to call latest_output_html and submit_command_html, include hx-headers to request text/html, and use hx-swap="innerHTML"/hx-swap="beforeend" to let HTMX perform native swaps.
• Remove the client-side JSON renderer and json-encoding extension by deleting html/app.js and html/vendor/json-enc-1.9.12.js, and add SQL/browser tests plus README updates that document the HTML RPC contract.

Testing

• Ran the full test suite with pytest -q, yielding 54 passed, 24 skipped where skipped tests require optional external services (database/PostgREST/Playwright) and are expected to be environment-dependent.
• All modified tests covering SQL rendering (tests/test_functions.py) and browser expectations (tests/test_html_ui.py) passed in the CI-like run.

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Codex Task