wip(k1): checkpoint connection recovery rewrite

Capture the current unreleased K1 connection, recovery, lifecycle, viewer, and test work as a single known-bad baseline for subsequent fixes.

src/k1link/web/frontend_assets.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+from fastapi.staticfiles import StaticFiles
+from starlette.responses import Response
+from starlette.types import Scope
+
+HTML_NO_STORE = "no-store"
+HASHED_ASSET_IMMUTABLE = "public, max-age=31536000, immutable"
+
+_MODULE_SCRIPT = re.compile(
+    r'<script\b[^>]*\bsrc=["\'](?P<src>/assets/[^"\']+)["\'][^>]*>',
+    re.IGNORECASE,
+)
+_HASHED_ASSET = re.compile(
+    r"^assets/(?:.+)-[A-Za-z0-9_-]{8,}\.[A-Za-z0-9]+$",
+)
+
+
+def frontend_build_id(frontend_root: Path) -> str | None:
+    """Return the exact content-hashed module loaded by the current index."""
+
+    try:
+        index = (frontend_root / "index.html").read_text(encoding="utf-8")
+    except OSError:
+        return None
+    for match in _MODULE_SCRIPT.finditer(index):
+        source = match.group("src")
+        if _HASHED_ASSET.fullmatch(source.removeprefix("/")):
+            return source
+    return None
+
+
+class ControlStationStaticFiles(StaticFiles):
+    """Serve the SPA shell fresh while retaining immutable hashed assets."""
+
+    async def get_response(self, path: str, scope: Scope) -> Response:
+        response = await super().get_response(path, scope)
+        if response.status_code >= 400:
+            return response
+        content_type = response.headers.get("content-type", "").lower()
+        normalized = path.lstrip("/")
+        if content_type.startswith("text/html"):
+            response.headers["Cache-Control"] = HTML_NO_STORE
+        elif _HASHED_ASSET.fullmatch(normalized):
+            response.headers["Cache-Control"] = HASHED_ASSET_IMMUTABLE
+        return response