test(cli): extract shared Fixture into tests/common

The single bare-metal integration test now reuses a `LazyLock<Fixture>`
that spawns picloud once on a private port and shares it across every
test in the binary. Sets the stage for per-surface journey modules
(auth, apps, scripts, invoke, logs, roles, output) without each one
paying for its own server spawn — same trick the dashboard Playwright
suite uses with global-setup.

Notes:
- `tests/cli.rs` becomes a tiny module list; the seed flow moved to
  `tests/integration.rs`. The seed slug now goes through
  `common::unique_slug` so parallel/serial reruns can't collide.
- `autotests = false` + an explicit `[[test]] name = "cli"` keeps Cargo
  from auto-promoting future `tests/*.rs` files into their own binaries
  (which would each respawn picloud).
- Subprocess cleanup uses `libc::atexit` to SIGTERM picloud when the
  test binary exits. PR_SET_PDEATHSIG was tried and rejected: it fires
  when the *thread* that forked dies, and cargo's per-test worker
  threads exit between tests, which killed the fixture mid-suite.
- New helpers: AppGuard/UserGuard (RAII teardown), member_user /
  grant_membership / update_membership (direct API for role tests),
  unique_slug / unique_username, pic_as / pic_no_env.
- Two `fixture_url_is_shared_*` tests prove the LazyLock is actually
  shared, not respawned per test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/picloud-cli/Cargo.toml

@@ -7,11 +7,20 @@ license.workspace = true
 repository.workspace = true
 authors.workspace = true
 description = "PiCloud command-line client"
+# Each top-level `tests/*.rs` would otherwise auto-discover as its own
+# test binary, respawning picloud once per file. We want one binary
+# with module sub-files (auth.rs, apps.rs, …) so the LazyLock fixture
+# is genuinely shared.
+autotests = false
 
 [[bin]]
 name = "pic"
 path = "src/main.rs"
 
+[[test]]
+name = "cli"
+path = "tests/cli.rs"
+
 [dependencies]
 picloud-shared.workspace = true
 reqwest = { workspace = true, features = ["json"] }
@@ -29,3 +38,4 @@ assert_cmd = "2"
 predicates = "3"
 tempfile = "3"
 reqwest = { workspace = true, features = ["json", "blocking"] }
+libc = "0.2"