feat(firmware): ESP32-C6 target — Wi-Fi 6 / 802.15.4 / TWT / LP-core (ADR-110)

`firmware/esp32-csi-node` now builds for both `esp32s3` (existing
production) and `esp32c6` (new research / battery-seed target) from
the same source tree. ESP-IDF auto-applies `sdkconfig.defaults.esp32c6`
when the target is set to esp32c6; every C6 module is gated on
CONFIG_IDF_TARGET_ESP32C6 (or the SOC_WIFI_HE_SUPPORT capability) so
the S3 build path is byte-identical to today.

New modules (all #ifdef-gated, no-op stubs on S3):
- c6_twt.{h,c}      — iTWT wrapper, graceful AP-NACK fallback
- c6_timesync.{h,c} — 802.15.4 beacon-based mesh time-sync, EUI-64
                      leader election, c6_timesync_get_epoch_us()
- c6_lp_core.{h,c}  — wake-on-motion deep-sleep helper (ext1 path
                      this cut; real LP-core polling deferred)

ADR-018 frame extension:
- byte 18: PPDU type (0=HT/legacy, 1=HE-SU, 2=HE-MU, 3=HE-TB)
- byte 19: bandwidth + STBC + 802.15.4-sync-valid flags
- Magic 0xC5110001 unchanged — backwards compatible
- Dual-branch encoding handles both struct variants of
  wifi_pkt_rx_ctrl_t (legacy S3 / HE C6) per CONFIG_SOC_WIFI_HE_SUPPORT

Critical bug fixed during live witness collection (verified across 3
boards on COM6/COM9/COM12):
- c6_timesync.c read MAC into a 6-byte buffer and ran MAC-48->EUI-64
  conversion. But esp_read_mac(ESP_MAC_IEEE802154) returns 8 bytes
  already in EUI-64 form on C6 — code was double-inserting FFFE.
  Boot log was 206ef1fffefffe17, fix yields 206ef1fffe17278c which
  matches esptool's eFuse reading exactly.

Tooling:
- CI workflow (firmware-ci.yml) extended with c6-4mb matrix row +
  ADR-110 host-unit-test step
- Host unit tests for pure functions (mac48_to_eui64,
  eui64_bytes_to_u64, PPDU encoding both branches) — runs on Ubuntu CI
- Multi-board live-capture harness (test/capture-3board-experiment.py)
- Witness bundle script records SHA-256s for s3-adr110, c6-adr110, and
  s3-fair-adr110 (apples-to-apples) binary archives

Honest empirical findings (full report in docs/WITNESS-LOG-110.md):
- Verified live on 3 C6 boards: boot, 802.15.4 init w/ correct EUIs,
  WiFi STA reaching assoc->run on ruv.net, TWT setup attempted +
  gracefully NACKed (AP is 11n-only, TWT Responder:0), HE-MAC firmware
  loaded
- NOT verified (need 11ax AP / second-channel exp / INA meter):
  HE-LTF subcarrier expansion, TWT cadence determinism, ±100 µs sync
  alignment, 5 µA hibernation
- Bug found: leader election doesn't step down under live WiFi load —
  likely 2.4 GHz radio coex preemption (WiFi ch 5 vs 15.4 ch 15);
  follow-up task #30
- Apples-to-apples size: S3-no-display = 886 KB, C6 = 1003 KB
  (C6 is 13% LARGER for equivalent CSI features; the extra is the
  802.15.4 + OpenThread stack that S3 lacks)

Tracking: ruvnet/RuView#762

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/firmware-ci.yml

@@ -38,7 +38,7 @@ jobs:
           echo "version.txt matches the release tag."
 
   build:
-    name: Build ESP32-S3 Firmware (${{ matrix.variant }})
+    name: Build firmware (${{ matrix.target }} / ${{ matrix.variant }})
     runs-on: ubuntu-latest
     container:
       image: espressif/idf:v5.4
@@ -47,17 +47,27 @@ jobs:
       matrix:
         include:
           - variant: 8mb
+            target: esp32s3
             sdkconfig: sdkconfig.defaults
             partition_table_name: partitions_display.csv
             size_limit_kb: 1100
             artifact_app: esp32-csi-node.bin
             artifact_pt: partition-table.bin
           - variant: 4mb
+            target: esp32s3
             sdkconfig: sdkconfig.defaults.4mb
             partition_table_name: partitions_4mb.csv
             size_limit_kb: 1100
             artifact_app: esp32-csi-node-4mb.bin
             artifact_pt: partition-table-4mb.bin
+          # ADR-110: ESP32-C6 research target (Wi-Fi 6 / 802.15.4 / TWT / LP-core)
+          - variant: c6-4mb
+            target: esp32c6
+            sdkconfig: sdkconfig.defaults
+            partition_table_name: partitions_4mb.csv
+            size_limit_kb: 1100
+            artifact_app: esp32-csi-node-c6.bin
+            artifact_pt: partition-table-c6.bin
 
     steps:
       - uses: actions/checkout@v4
@@ -66,12 +76,22 @@ jobs:
         working-directory: firmware/esp32-csi-node
         run: |
           . $IDF_PATH/export.sh
-          if [ "${{ matrix.variant }}" != "8mb" ]; then
+          # 4mb variant supplies its own sdkconfig.defaults overlay.
+          # c6-4mb variant relies on the auto-applied sdkconfig.defaults.esp32c6
+          # overlay (ESP-IDF auto-loads sdkconfig.defaults.$TARGET when present).
+          if [ "${{ matrix.variant }}" = "4mb" ]; then
             cp "${{ matrix.sdkconfig }}" sdkconfig.defaults
           fi
-          idf.py set-target esp32s3
+          idf.py set-target ${{ matrix.target }}
           idf.py build
 
+      - name: Build and run host-side ADR-110 unit tests
+        if: matrix.variant == 'c6-4mb'
+        working-directory: firmware/esp32-csi-node/test
+        run: |
+          make test_adr110
+          ./test_adr110
+
       - name: Verify binary size (< ${{ matrix.size_limit_kb }} KB gate)
         working-directory: firmware/esp32-csi-node
         run: |