task-39: the in-call speaker button routes to a headset, and why

gnome-calls asks org.mobian_project.CallAudio, which callaudiod owns, and
callaudiod switches a sink port - which cannot work when ACP splits earpiece and
speaker across two profiles. The working toggle is the hook's speaker on|off and
nothing in the UI calls it. Fix is a shim that claims the name; it needs a real
call to test before enabling.

docs/tasks/39-audio-levels-as-state.md

@@ -172,3 +172,42 @@ which gives a flat noise floor at rms ~14000 with no speech variation at all;
 84 drops it to ~6200. A runtime `amixer cset` does **not** persist — activating
 the profile reapplies the UCM value, verified. Whatever gain STT wants has to
 land in `souveraine-ucm-blueline`.
+
+## The in-call speaker button routes to a headset (root cause, 2026-08-02)
+
+Casey, observed: an inbound call **defaulted to speakerphone**, and tapping the
+speaker button (to *turn on* speakerphone) **switched to headset mode**.
+
+Not mysterious once the ownership is traced:
+
+- `gnome-calls` is the dialer, and its speaker button calls the D-Bus name
+  `org.mobian_project.CallAudio`.
+- That name is owned by upstream **`callaudiod`**
+  (`/usr/share/dbus-1/services/org.mobian_project.CallAudio.service`,
+  `Exec=/usr/bin/callaudiod`), which is D-Bus activated on demand.
+- **callaudiod cannot route this card.** It switches a sink *port* and needs a
+  speaker port and an earpiece port on one sink; ACP models UCM devices as one
+  profile per combination — `Voice Call (Earpiece, Mic)` and
+  `Voice Call (Mic, Speaker)` — so no sink carries both. `blueline-callaudio-hook`
+  already says this in a comment; the consequence was never written down.
+- So the button asks for a port that does not exist and lands on a headset
+  route. **The working toggle already exists and nothing in the UI calls it:**
+  `blueline-callaudio-hook speaker on|off` does direct mixer writes and is
+  verified working (`speaker: on` / `speaker: off (earpiece)`).
+
+`souveraine-callaudio` ships only the hook — it does **not** contest the D-Bus
+name, so callaudiod always wins.
+
+**The fix is a shim that claims `org.mobian_project.CallAudio`** and translates
+`EnableSpeaker`/`SelectMode` into the hook's `speaker on|off`. It must be tested
+against a real inbound call before it is enabled: this path is phone calls, and
+a broken shim loses them entirely.
+
+Until then the manual escape is `sudo blueline-callaudio-hook speaker on`.
+
+Note the default-to-speakerphone half is a *separate* symptom: `apply()` forces
+Earpiece deliberately ("enabling Speaker here is what made every call come out
+of the loudspeaker with no way back"), and the hook is enabled, active and
+unshadowed (`FragmentPath=/usr/lib/systemd/system/...`). So something else put
+the card in a speaker profile before or during that call — unreproduced, and it
+needs a call to chase.