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.cpp

@@ -67,11 +67,12 @@ static bool audio10_set_req_entity(tusb_control_request_t const *p_request, uint
                         TU_VERIFY(p_request->wLength == 2);
 
                         volume[index] = static_cast<float>(*reinterpret_cast<int16_t const *>(pBuff)) / 256;
-                        if (entityID == UAC1_ENTITY_SPK_FEATURE_UNIT) {
-                            auto config = get_config();
-                            config.speaker_volume = volume[index];
-                            set_config(config);
-                        }
+                        // Do NOT sync the USB-side UAC1 volume control into our
+                        // flash-persisted config. The host (PipeWire/Pulse) re-applies
+                        // its last-known UAC1 volume on every device reconnect, which
+                        // would silently override the user's saved speaker_volume.
+                        // Fix borrowed from loteran/DS5Dongle commit 03fa1e4.
+                        (void)entityID;
 
                         TU_LOG2("    Set Volume: %d dB of entity: %u\r\n", volume[index], entityID);
                         return true;