Bluetooth mic startup: honest Listening indicator, startup-time investigation

The bar turned green after 2 s on a timer even when the microphone delivered
nothing. It now turns green only on real audio: RMS above 0.0005, or three
consecutive chunks that are not digital silence (the virtual mic emits one
stray nonzero chunk right after start, and a Bluetooth headset's floor ramps
in from a few LSB). Measured 1.2-1.4 s after the key, within 0.14 s of the
first real samples.

diagnostics/bluetooth/ records how startup went from ~1.6 s (or never) to
~1.1-1.4 s: a btusb driver bug, two PipeWire bluez5 bugs, a 500 ms WirePlumber
switch timeout and an over-broad auto-connect rule. Those fixes are machine
level and live outside this repo; the patches, tools and measurements are here.
STARTUP-TIME.md is the summary and explains the ~0.9 s hardware floor.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xoc9DJCViR7dg9eKzzAcfQ

diagnostics/bluetooth/STARTUP-TIME.md

@@ -0,0 +1,55 @@
+# How we cut Bluetooth microphone startup time
+
+Measured on a Sony WH-1000XM5 over an Intel AX210, PipeWire 1.6.8, WirePlumber
+0.5.17, kernel 7.2.3. Times are after the dictation key press.
+
+| | before | after |
+|---|---|---|
+| mic link (eSCO) up | 0.7–1.0 s, sometimes never | 0.26–0.49 s |
+| first real audio at the daemon | 1.5–1.7 s, or never | 1.06–1.43 s |
+| "Listening" in the bar | 2.1 s on a timer, even with no audio | 1.19–1.43 s, only on real audio |
+
+## What changed
+
+| Change | Where it lives | Gain |
+|---|---|---|
+| Daemon indicator: green on real audio only (RMS > 0.0005 or three consecutive non-silent chunks), no timer | `daemon/sttd.py` | the bar cannot claim to listen to a dead mic |
+| WirePlumber profile-switch timeout 500 ms → 50 ms | `~/.local/share/wireplumber/scripts/device/autoswitch-bluetooth-profile.lua` | link up ~0.45 s sooner |
+| A2DP auto-connect rule narrowed to the XM5 | `~/.config/wireplumber/wireplumber.conf.d/bluetooth-a2dp-autoconnect.conf` | no 5 s pages of absent devices at login |
+| btusb driver patch `0002` | `/usr/lib/modules/<kernel>/updates/btusb.ko` | mic link no longer comes up silent when a page overlaps it |
+| PipeWire bluez5 patches `0001` + `0003` | `~/.local/lib/spa-0.2/bluez5/` via `SPA_PLUGIN_DIR` drop-ins | no stale-error dead takes; back-to-back takes work |
+
+Only the first row is part of this repo. The rest are machine-level and are
+described, with patches and measurements, in [HANDOFF.md](HANDOFF.md).
+
+## Why ~0.9 s is the floor
+
+```
+key press
+0.00 s ─┬─ daemon starts pw-record on camera-effects-mic
+        │
+        │   Camera Effects notices the client (250 ms poll),
+        │   opens its helper stream, WirePlumber waits 50 ms,
+        │   then asks BlueZ for the headset profile
+        │
+0.30 s ─┼─ eSCO link up (the headset must leave sniff mode,
+        │   then the radio sets up the synchronous link: ~0.1 s)
+        │
+        │   headset sends encoded digital silence while its own
+        │   mic path starts and mSBC syncs: ~0.5–0.6 s
+        │   (measured at the decoder; Linux cannot shorten it)
+        │
+0.85 s ─┼─ first non-zero samples leave the headset
+        │   Camera Effects processing + 50 ms chunking
+0.90 s ─┴─ first real audio reaches the daemon
+```
+
+What is left on the Linux side is the ~0.25 s before the profile switch is
+even requested. The daemon could ask for the headset profile itself when
+capture starts, but the WirePlumber autoswitch script only restores a profile
+it switched, so the daemon would have to restore A2DP too. Not done.
+
+The headset's silence after link-up and the eSCO setup are hardware. Words
+spoken in the first ~0.9 s are lost unless the microphone is already in
+hands-free mode, which drops playback to mono, so it is off by default
+(`warmMic`).