fix(harness): judge the install separately from backend readiness

The first run against the branch's own code confirmed the doctor fix — no
traceback, the firecracker probes now report "TAP pool: 0/8" instead of dying
on PermissionError — and then failed for a reason the installer cannot fix.

The Apple `container` service is per-USER. `container system status` reports an
appRoot under ~/Library/Application Support/com.apple.container/, and bbtest
sees "container system service: NOT running" while the creating admin's is
running. A brand-new account therefore has no backend until it runs
`container system start` once, doctor rightly fails, and `test` would be
permanently red for host state that install.sh neither creates nor can regress.

So stop treating doctor's exit code as one verdict. `test` now fails when the
install is broken — no entry point, doctor crashed, or doctor could not report
a usable python and config dir — and reports backend readiness as a note.
BB_TEST_REQUIRE_BACKEND=1 restores the strict behaviour.

A traceback is checked for explicitly rather than inferred from the exit code,
because those are the same value. The PermissionError bug exited non-zero
exactly like a missing prerequisite does; only the traceback distinguishes a
defect from an environment fact, and that distinction is the whole point of
this change.

The research doc's footprint table had the service as a host-wide launchd
service that survives user deletion. It does not. The state lives in the home
and goes with it, so the reset is more complete than claimed — but the
corollary is that a fresh account cannot run a bottle until the service is
started for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WEfZZhakx13bxTfXcZCoS5

docs/research/testing-clean-install-on-macos.md

@@ -72,14 +72,24 @@ Grounding the teardown story in what `install.sh` and the backend create:
 | private venv fallback (no pipx) | `~/.bot-bottle/venv` + symlink in `~/.local/bin` ([`install.sh`](../install.sh)) | ✅ | ❌ removed with home |
 | PATH / token exports | shell profile (`~/.zprofile`, etc.); `BOT_BOTTLE_CLAUDE_OAUTH_TOKEN` ([`README.md:74`](../README.md)) | ✅ | ❌ removed with home |
 | **Apple `container` install** | `/usr/local/...` + notarized `.pkg` receipts | ❌ | ✅ **stays** |
-| **Apple `container` system service** | launchd system service (`container system start`) | ❌ | ✅ **stays** |
+| Apple `container` **service state** | per-user: `~/Library/Application Support/com.apple.container/` (`container system start`) | ✅ | ❌ removed with home |
 | **Homebrew** (if used for `container`/python) | `/opt/homebrew` | ❌ | ✅ **stays** |
 | Rosetta 2 (needed for image builds) | system | ❌ | ✅ **stays** |
 
-The three bold rows are the crux: **deleting the throwaway user does not
-uninstall the Apple Container runtime, its system service, Homebrew, or
-Rosetta.** A VM, by contrast, wipes 100% of the above by definition —
-that's its entire advantage for this task.
+The bold rows are the crux: **deleting the throwaway user does not uninstall
+the Apple Container runtime, Homebrew, or Rosetta.** A VM, by contrast, wipes
+100% of the above by definition — that's its entire advantage for this task.
+
+The service row is the exception, and a live harness run corrected it: the
+Apple `container` service is **per-user**, not a host-wide launchd service.
+`container system status` reports an `appRoot` under
+`~/Library/Application Support/com.apple.container/`, and a freshly created
+account sees `container system service: NOT running` even while the creating
+admin's is running. That cuts both ways — the state is genuinely removed with
+the home, so the reset is *more* complete than this table first claimed, but
+it also means **no brand-new user can run a bottle until they run
+`container system start` once**. `doctor` correctly fails until they do, which
+is why `test` judges backend readiness separately from install correctness.
 
 ## Option A — Disposable Tart VM (recommended)