feat: UWB anchor auto-discovery (Issue #698)

Replace hardcoded NUM_ANCHORS with dynamic discovery table.
Tag probes IDs 0..7 via standard DS-TWR POLL frames at startup
and after every DISC_RESCAN_MS (10 s); anchors that respond are
added to the active table, stale ones removed after
ANCHOR_MISS_LIMIT misses or ANCHOR_TIMEOUT_MS silence.

Key changes
-----------
- AnchorEntry table (8 slots): present/active flags, range_mm,
  rssi, last_ok_ms, miss_count
- DISC_SCAN → DISC_RANGE state machine
- range_and_update(): ranges, updates table, emits +DISC:FOUND /
  +DISC:LOST on serial and MSG_DISC (0x40) on ESP-NOW
- disc_scan_step(): advances cursor one probe per ranging slot,
  skips already-active IDs
- New OLED screens: ANCHOR SCAN progress bar, NO ANCHOR rescan
  countdown, active anchor count badge on normal screen
- platformio.ini: switch to MaUWB_DW3000 lib, remove NUM_ANCHORS
  build flag (discovery is now dynamic)
- No anchor firmware changes required

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

esp32/uwb_tag/platformio.ini

@@ -1,8 +1,17 @@
-; SaltyBot UWB Tag Firmware
+; SaltyBot UWB Tag Firmware — Issue #545, Issue #698
-; Target: Makerfabs ESP32 UWB Pro with Display (DW1000 + SSD1306 OLED)
+; Target: Makerfabs ESP32 UWB Pro with Display (DW3000 + SSD1306 OLED)
+;
+; The tag is battery-powered, worn by the person being tracked.
+; It initiates DS-TWR ranging with each anchor in round-robin,
+; shows status on OLED display, and sends data via ESP-NOW.
+;
+; Library: Makerfabs MaUWB_DW3000
+;   https://github.com/Makerfabs/MaUWB_DW3000
 ;
 ; Flash:
 ;   pio run -e tag --target upload
+; Monitor (USB debug):
+;   pio device monitor -b 115200
 
 [env:tag]
 platform      = espressif32
@@ -10,12 +19,11 @@ board         = esp32dev
 framework     = arduino
 monitor_speed = 115200
 upload_speed  = 921600
-lib_extra_dirs = ../../lib
 lib_deps =
+    https://github.com/Makerfabs/MaUWB_DW3000.git
     adafruit/Adafruit SSD1306@^2.5.7
     adafruit/Adafruit GFX Library@^1.11.5
-board_build.partitions = huge_app.csv
 build_flags =
     -DCORE_DEBUG_LEVEL=0
-    -DTAG_ID=0x01
+    -DTAG_ID=0x01        ; unique per tag (0x01–0xFE)
-    -Wno-error=return-type
+    -DRANGE_INTERVAL_MS=50  ; 20 Hz round-robin across discovered anchors