test(library): read one "now" in the fixture and the page across midnight

The concept fixture recorded each prompt's last use with the database's wall
clock at seeding, while the page computed "today" from its own clock at
render. A run seeded at 23:59:5x UTC and rendered after midnight read "Used
yesterday" against the concept's "Used today" (PR #284, frontend coverage
shard 2/3).

The concept fixture now takes one instant (optional "now" through the
concept reset hook, the wall clock otherwise) for every last use, mtime and
run time it writes and returns it as manifest.now; seedConceptAt fixes the
browser's Date at that instant. The Library and Images concept specs seed
through it, and the two workspace specs that assert "Used today" and the
"Today" caption fix the page clock at the time they patch or seed. A new
test starts the browser at 23:59:58 UTC, seeds at that instant and passes
midnight before either document opens.

tests/browser/server.py

@@ -22,6 +22,7 @@ from tests.browser.workspace_activity_fixture import register_activity_commands
 from tests.browser.workspace_concept_fixture import (
     MAX_EXTRA_IMAGES as CONCEPT_MAX_EXTRA_IMAGES,
     MAX_SEEDLESS_IMAGES as CONCEPT_MAX_SEEDLESS_IMAGES,
+    MAX_EPOCH_SECONDS as CONCEPT_MAX_EPOCH_SECONDS,
     seed_workspace_concept,
 )
 from tests.browser.workspace_visual_fixture import seed_workspace_visual
@@ -486,8 +487,17 @@ def create_app(root: Path) -> web.Application:
         if (
             request.query
             or not isinstance(data, dict)
-            or set(data) - set(limits) - {"short_seedless"}
+            or set(data) - set(limits) - {"short_seedless", "now"}
             or not isinstance(data.get("short_seedless", False), bool)
+            # A chosen "now" (epoch seconds) pins every time the fixture writes.
+            or (
+                "now" in data
+                and (
+                    isinstance(data["now"], bool)
+                    or not isinstance(data["now"], (int, float))
+                    or not 0 < data["now"] < CONCEPT_MAX_EPOCH_SECONDS
+                )
+            )
             or any(
                 isinstance(data.get(key, 0), bool)
                 or not isinstance(data.get(key, 0), int)
@@ -507,6 +517,7 @@ def create_app(root: Path) -> web.Application:
                 extra_images=data.get("extra_images", 0),
                 seedless_images=data.get("seedless_images", 0),
                 short_seedless=data.get("short_seedless", False),
+                now=data.get("now"),
             )
         )
         return web.json_response({"success": True, **manifest})