Phase 4: Jinja bridge for incremental s-expression migration

Two-way bridge: sexp() Jinja global renders s-expression components in
templates, register_components() loads definitions at startup. Includes
~link-card component test proving unified replacement of 5 per-service
Jinja fragment templates.

19 new tests (218 total).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sexp/jinja_bridge.py

@@ -0,0 +1,138 @@
+"""
+Jinja ↔ s-expression bridge.
+
+Provides two-way integration so s-expression components and Jinja templates
+can coexist during incremental migration:
+
+**Jinja → s-expression** (use s-expression components inside Jinja templates)::
+
+    {{ sexp('(~link-card :slug "apple" :title "Apple")') | safe }}
+
+**S-expression → Jinja** (embed Jinja output inside s-expressions)::
+
+    (raw! (jinja "fragments/link_card.html" :slug "apple" :title "Apple"))
+
+Setup::
+
+    from shared.sexp.jinja_bridge import setup_sexp_bridge
+    setup_sexp_bridge(app)  # call after setup_jinja(app)
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from .types import NIL, Symbol
+from .parser import parse
+from .html import render as html_render
+
+
+# ---------------------------------------------------------------------------
+# Shared component environment
+# ---------------------------------------------------------------------------
+
+# Global component registry — populated at app startup by loading component
+# definition files or calling register_components().
+_COMPONENT_ENV: dict[str, Any] = {}
+
+
+def get_component_env() -> dict[str, Any]:
+    """Return the shared component environment."""
+    return _COMPONENT_ENV
+
+
+def register_components(sexp_source: str) -> None:
+    """Parse and evaluate s-expression component definitions into the
+    shared environment.
+
+    Typically called at app startup::
+
+        register_components('''
+            (defcomp ~link-card (&key link title image icon)
+              (a :href link :class "block rounded ..."
+                (div :class "flex ..."
+                  (if image
+                    (img :src image :class "...")
+                    (div :class "..." (i :class icon)))
+                  (div :class "..." (div :class "..." title)))))
+        ''')
+    """
+    from .evaluator import _eval
+    from .parser import parse_all
+
+    exprs = parse_all(sexp_source)
+    for expr in exprs:
+        _eval(expr, _COMPONENT_ENV)
+
+
+# ---------------------------------------------------------------------------
+# sexp() — render s-expression from Jinja template
+# ---------------------------------------------------------------------------
+
+def sexp(source: str, **kwargs: Any) -> str:
+    """Render an s-expression string to HTML.
+
+    Keyword arguments are merged into the evaluation environment,
+    so Jinja context variables can be passed through::
+
+        {{ sexp('(~link-card :title title :slug slug)',
+                title=post.title, slug=post.slug) | safe }}
+
+    This is a synchronous function — suitable for Jinja globals.
+    For async resolution (with I/O primitives), use ``sexp_async()``.
+    """
+    env = dict(_COMPONENT_ENV)
+    env.update(kwargs)
+    expr = parse(source)
+    return html_render(expr, env)
+
+
+async def sexp_async(source: str, **kwargs: Any) -> str:
+    """Async version of ``sexp()`` — resolves I/O primitives (frag, query)
+    before rendering.
+
+    Use when the s-expression contains I/O nodes::
+
+        {{ sexp_async('(frag "blog" "card" :slug "apple")') | safe }}
+    """
+    from .resolver import resolve, RequestContext
+
+    env = dict(_COMPONENT_ENV)
+    env.update(kwargs)
+    expr = parse(source)
+
+    # Try to get request context from Quart
+    ctx = _get_request_context()
+    return await resolve(expr, ctx=ctx, env=env)
+
+
+def _get_request_context():
+    """Build RequestContext from current Quart request, if available."""
+    from .primitives_io import RequestContext
+    try:
+        from quart import g, request
+        user = getattr(g, "user", None)
+        is_htmx = bool(request.headers.get("HX-Request"))
+        return RequestContext(user=user, is_htmx=is_htmx)
+    except Exception:
+        return RequestContext()
+
+
+# ---------------------------------------------------------------------------
+# Quart integration
+# ---------------------------------------------------------------------------
+
+def setup_sexp_bridge(app: Any) -> None:
+    """Register s-expression helpers with a Quart app's Jinja environment.
+
+    Call this in your app factory after ``setup_jinja(app)``::
+
+        from shared.sexp.jinja_bridge import setup_sexp_bridge
+        setup_sexp_bridge(app)
+
+    This registers:
+    - ``sexp(source, **kwargs)`` — sync render (components, pure HTML)
+    - ``sexp_async(source, **kwargs)`` — async render (with I/O resolution)
+    """
+    app.jinja_env.globals["sexp"] = sexp
+    app.jinja_env.globals["sexp_async"] = sexp_async