feat(cache): serve every checkout's own cache declarations

A checkout that declared a new cache host rendered the artefacts and reached nothing: the Nexus proxy repository, the frontend's leaf certificate and the nginx upstream map all live in the running stack, which only the owning checkout set up. cache-apply installs all three and reloads the frontend, and compose up runs it for a checkout that merely joins the network, so the manual call is left for a declaration changed between two bring-ups.

The map source is no longer selectable. INFINITO_CACHE_CONF_SOURCE existed to let a worktree read the primary checkout's map instead of its own, but the frontend mounts one map and the install replaces it, so the checkout that applied last wins either way: the knob chose which file to point at, never who prevailed. Dropping it also retires the git subprocess that resolved the owning checkout on every env build.

Two checkouts sharing a stack therefore overwrite each other's map, which the documentation now states instead of implying a selector could prevent it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/python/cli/administration/deploy/development/test_compose.py

@@ -295,6 +295,41 @@ class TestComposeUpRetries(unittest.TestCase):
         compose._generate_package_frontend_certs.assert_called_once()
         compose._install_package_frontend_ca_in_runner.assert_called_once()
 
+    @patch.dict(
+        os.environ,
+        {
+            "INFINITO_BUILD": "1",
+            "INFINITO_IMAGE": "infinito-debian",
+            "INFINITO_PULL_POLICY": "never",
+            "CI": "",
+            "GITHUB_ACTIONS": "",
+            "INFINITO_RUNNING_ON_GITHUB": "",
+            "INFINITO_GIT_COMMON_DIR": "/primary/.git",
+            "INFINITO_CACHE_NETWORK": "infinito-nexus-core_default",
+            "INFINITO_CACHE_STACK": "",
+        },
+        clear=False,
+    )
+    def test_up_applies_the_declarations_to_a_cache_stack_it_shares(self) -> None:
+        compose = self._compose()
+        compose._render_coredns_corefile = MagicMock()
+        compose._compose_up_with_retries = MagicMock()
+        compose.wait_for_healthy = MagicMock()
+        compose._bootstrap_package_cache = MagicMock()
+        compose._generate_package_frontend_certs = MagicMock()
+        compose._apply_shared_cache = MagicMock()
+        compose._install_package_frontend_ca_in_runner = MagicMock()
+
+        compose.up(run_entry_init=False)
+
+        compose._compose_up_with_retries.assert_called_once_with(
+            ["up", "-d", "coredns", "infinito"], attempts=6, delay_s=30
+        )
+        compose._generate_package_frontend_certs.assert_not_called()
+        compose._bootstrap_package_cache.assert_not_called()
+        compose._apply_shared_cache.assert_called_once()
+        compose._install_package_frontend_ca_in_runner.assert_called_once()
+
 
 if __name__ == "__main__":  # pragma: no cover
     unittest.main()