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>

CMakeLists.txt

@@ -88,6 +88,8 @@ add_executable(ds5-bridge
         src/config.cpp
         src/cmd.cpp
         src/state_mgr.cpp
+        src/oled.cpp
+        src/slots.cpp
 )
 
 if (ENABLE_BATT_LED)
@@ -144,7 +146,8 @@ if(WAKE_DEBUG)
 endif()
 set(VERSION "dev" CACHE STRING "Program version string")
 
-pico_set_program_name(ds5-bridge "ds5-bridge")
+set_target_properties(ds5-bridge PROPERTIES OUTPUT_NAME "ds5-bridge-oled")
+pico_set_program_name(ds5-bridge "ds5-bridge-oled")
 pico_set_program_version(ds5-bridge "${VERSION}")
 
 # Modify the below lines to enable/disable output over UART/USB
@@ -167,6 +170,7 @@ target_link_libraries(ds5-bridge
         hardware_watchdog
         hardware_timer
         hardware_flash
+        hardware_spi
         pico_btstack_classic
 #        pico_cyw43_arch_threadsafe_background
         pico_cyw43_arch_poll