release(firmware): v0.6.9-esp32 — sync-packet wired, CONFIG_C6_SYNC_EVERY_N_FRAMES tunable

Bundles the iter 8 + iter 9 sync-packet work (§A0.11 + §A0.12) into a
shipped release. v0.6.8 didn't carry the sync emission; v0.6.9 closes
the loop.

What ships:
- csi_collector emits a 32-byte UDP sync packet (magic 0xC511A110)
  every CONFIG_C6_SYNC_EVERY_N_FRAMES CSI callbacks (default 20).
- New Kconfig knob lets operators tune cadence from ~0.1 Hz (N=1000)
  to ~10 Hz (N=1) without rebuilding — sensible defaults for
  mainstream multistatic at ~2 s sync interval.
- Backwards-compatible at the wire level: old aggregators drop the new
  magic on existing parser mismatch path.

Build artifacts (both green on IDF v5.4):
- S3 8 MB: 1094 KB, 47% partition slack
- C6 4 MB: 1019 KB, 45% partition slack

The macro define was renamed from SYNC_EVERY_N_FRAMES to
CONFIG_C6_SYNC_EVERY_N_FRAMES so the Kconfig generator wires through.
Header guard preserves the default for builds without the kconfig
applied.

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

firmware/esp32-csi-node/main/Kconfig.projbuild

@@ -402,6 +402,20 @@ menu "ESP32-C6 capabilities (ADR-110)"
         range 1 13
         depends on C6_SOFTAP_HE_ENABLE
 
+    config C6_SYNC_EVERY_N_FRAMES
+        int "Sync-packet emission cadence (CSI frames per sync)"
+        default 20
+        range 1 1000
+        help
+            How many CSI callbacks fire before csi_collector emits one
+            ADR-110 §A0.11 sync packet (magic 0xC511A110) carrying the
+            mesh-aligned epoch + sequence high-water for the host
+            aggregator to pair against incoming CSI frames.
+
+            Default 20 = ~2 s between sync packets at the bench's
+            observed 10 fps CSI rate. Raise for less wire overhead;
+            lower for tighter multistatic alignment windows.
+
 endmenu
 
 menu "ADR-018 frame extensions (ADR-110)"