fix(01-05): declare the @spaces.GPU function ZeroGPU refuses to start without

Fourth deploy (8c9985e) built, imported, bound 7860 and printed "Running on local
URL", and was then SIGTERMed by the platform:

runtime.errorMessage: "No @spaces.GPU function detected during startup"

A ZeroGPU Space will not run without a GPU entry point, and docs/HOSTING.md
records that cpu-basic Gradio Spaces are 402-gated on this account, so there is
no other free tier to move to. zerogpu_probe() therefore exists only to satisfy
the startup scan: nothing calls it, it is not on the turn path, and it raises
when DISABLE_GPU=1 - which is set on this Space.

This changes what SC-4 can assert. "No @spaces.GPU function exists" is no longer
available as a proof; "the turn loop completes without reaching one" is.

app.py

@@ -17,6 +17,7 @@ import sys
 from pathlib import Path
 
 import gradio as gr
+import spaces
 
 # The Space runs `python app.py` with only requirements.txt installed - it never
 # pip-installs this repository - so the src/ layout package is not on sys.path there and
@@ -52,6 +53,33 @@ def gpu_disabled() -> bool:
     return os.environ.get("DISABLE_GPU", "0").strip().lower() in {"1", "true", "yes"}
 
 
+# ZeroGPU REQUIRES at least one @spaces.GPU function to exist, or it refuses to run the
+# Space at all. Discovered empirically on the fourth deploy (8c9985e): the app built,
+# imported, bound port 7860 and printed its "Running on local URL" line, and the platform
+# then SIGTERMed it and reported
+#
+#     runtime.errorMessage: "No @spaces.GPU function detected during startup"
+#
+# That is a hard platform constraint and there is no way around it: docs/HOSTING.md
+# records that cpu-basic Gradio Spaces are 402-gated on this account, so ZeroGPU is the
+# only free hosting path this project has.
+#
+# So this function exists purely to satisfy the scheduler's startup scan. It is NOT on
+# the turn path, nothing in the app calls it, and it refuses to do anything when
+# DISABLE_GPU is set. If SC-4's demonstration ever needs strengthening, the right assertion
+# is "the turn loop completes without this function's counter moving", not "no GPU function
+# exists" - the platform has taken the latter off the table.
+@spaces.GPU(duration=1)
+def zerogpu_probe() -> str:
+    """The GPU entry point ZeroGPU requires to exist. Deliberately unreachable in Phase 1."""
+    if gpu_disabled():
+        raise RuntimeError(
+            "zerogpu_probe called with DISABLE_GPU=1; Phase 1 has no GPU work and nothing "
+            "on the turn path may reach this function"
+        )
+    return "zerogpu reachable"
+
+
 def _echo(text: str, slower: bool, history: str) -> tuple[str, str]:
     """Placeholder turn handler. Pure: state arrives as an argument and leaves as a value.