rebase: Phase B — trivial files for the v0.6.0 base

Build-system, config, usb, and CI wiring on top of upstream's
v0.6.0-hotfix tree. Build will still fail until Phase C+D land bt.h
accessors that oled.cpp references; that's expected mid-rebase.

CMakeLists.txt: add src/oled.cpp + src/slots.cpp to add_executable,
link hardware_spi, restore OUTPUT_NAME ds5-bridge-oled.

src/config.h: append the 4 OLED Edition Config_body fields
(current_slot + auto_haptics_enable/gain/lowpass). Preserves
upstream's field order for the first 8 fields.

src/config.cpp:
- speaker_volume validity default -100 dB → 0 dB (footgun fix)
- 4 new validity guards for the appended fields with sensible
  defaults (Fallback / 100 % / 160 Hz / slot 0)
- config_default() impl (was declared in upstream's config.h since
  v0.5.4 but never defined; the OLED Edition's "Reset to defaults"
  Settings item calls it)

src/usb.cpp: drop the UAC1 SET_CUR Volume → flash-config sync that
let PipeWire/Pulse silently override the user's saved Spk Vol on
every device reconnect. Borrowed from loteran/DS5Dongle commit
03fa1e4.

src/usb_descriptors.cpp: restore iSerialNumber = STRID_SERIAL.
Upstream's commit e79c762 zeroed it to work around SpecialK (#32),
but that broke Windows device identity (#100) — users lost
per-device volume / app preferences when moving USB ports. The
OLED Edition prioritizes Windows device-identity stability for
the broader user base over SpecialK compat for a narrower one.

.github/workflows/{build,release}.yml: artifact filenames use
ds5-bridge-oled.uf2 (and -debug / -picow variants) per the
OUTPUT_NAME rebrand.

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

src/usb_descriptors.cpp

@@ -102,7 +102,15 @@ tusb_desc_device_t desc_device =
 
     .iManufacturer = 0x01,
     .iProduct = 0x02,
-    .iSerialNumber = 0x00,
+    // OLED Edition restores the per-board serial number that upstream stripped
+    // in commit e79c762 ("remove usb serialnumber #32"). Without it, Windows
+    // treats the dongle as a NEW device for each USB port (issue awalol#100):
+    // users lose their per-device volume / app preferences when they move
+    // ports. The trade-off (re-introducing SpecialK incompatibility from #32)
+    // affects only Windows users with that specific tool; the broader Windows
+    // population gets stable device identity back. The serial string itself
+    // is produced by board_usb_get_serial() from the flash chip's unique ID.
+    .iSerialNumber = STRID_SERIAL,
 
     .bNumConfigurations = 0x01
 };