Phase 8.2: kernel ↔ daemon chardev bridge with round-trip test

Adds /dev/daedalus-v4l2 misc chardev to the kernel module. The
chardev is the IPC channel for the future userspace decoder
daemon: kernel enqueues REQ_* messages, daemon read()s them,
processes, write()s RESP_* back.

Wire protocol (pre-1.0, header in include/daedalus_v4l2_proto.h):
- struct daedalus_msg_hdr: magic (D04V) + version + type +
  cookie + payload_len + reserved
- Request/response separated by high bit of type field
- Max 64 KiB payload per message
- Cookie correlates request with matching response

Kernel implementation (kernel/daedalus_v4l2_chardev.{c,h}):
- Single-instance chardev (-EBUSY on second open)
- In-kernel FIFO bounded at 64 messages
- Blocking + non-blocking read; poll() with EPOLLIN on queued
- write() parses + validates header, logs response at pr_debug
- Bad magic → -EBADMSG, bad version → -EPROTO, oversize → -EMSGSIZE
- All error paths free resources

Phase 8.2 test trigger via debugfs:
- /sys/kernel/debug/daedalus_v4l2/test_ping — any byte
  enqueues a PING with a fixed 24-byte payload. Removed in
  Phase 8.4 when real REQ_DECODE from V4L2 path takes over.

Userspace verification tool (tools/test_chardev_pingpong.c):
- Real C program, proper error reporting via strerror
- Validates the 6-step round-trip: open → empty-queue EAGAIN →
  trigger ping → read PING → verify all fields → write PONG → close
- Builds with -Wall -Wextra clean

Verification on hertz (Pi 5, 6.12.75+rpt-rpi-2712):
  $ sudo insmod daedalus_v4l2.ko
  $ sudo tools/test_chardev_pingpong
  opening /dev/daedalus-v4l2...
    non-blocking read on empty queue: EAGAIN ✓
    injected PING via debugfs ✓
    read PING: magic ✓ version ✓ type=PING ✓ cookie=0x1234 ✓ payload=24 bytes
      payload: "DAEDALUS-V4L2-PING-PL"
    wrote PONG (cookie=0x1234) ✓
  ALL TESTS PASSED.
  $ sudo rmmod daedalus_v4l2      # clean

Per correctness-before-speed: full kerneldoc on structs, 8-tab
kernel style, SPDX headers, proper error paths, real test
program (not "I ran it once"), failure-mode coverage documented.

Phase 8.3 next: userspace daemon with dlopen'd FFmpeg parse path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

kernel/daedalus_v4l2_main.c

@@ -28,6 +28,8 @@
 #include <media/v4l2-dev.h>
 #include <media/v4l2-ioctl.h>
 
+#include "daedalus_v4l2_chardev.h"
+
 #define DAEDALUS_DRV_NAME	"daedalus_v4l2"
 #define DAEDALUS_VIDEO_NAME	"daedalus"
 
@@ -181,29 +183,40 @@ static int __init daedalus_init(void)
 {
 	int ret;
 
+	ret = daedalus_chardev_init();
+	if (ret)
+		return ret;
+
 	daedalus_platform_device = platform_device_alloc(DAEDALUS_DRV_NAME, -1);
-	if (!daedalus_platform_device)
-		return -ENOMEM;
+	if (!daedalus_platform_device) {
+		ret = -ENOMEM;
+		goto err_chardev;
+	}
 
 	ret = platform_device_add(daedalus_platform_device);
 	if (ret) {
 		platform_device_put(daedalus_platform_device);
-		return ret;
+		goto err_chardev;
 	}
 
 	ret = platform_driver_register(&daedalus_platform_driver);
 	if (ret) {
 		platform_device_unregister(daedalus_platform_device);
-		return ret;
+		goto err_chardev;
 	}
 
 	return 0;
+
+err_chardev:
+	daedalus_chardev_exit();
+	return ret;
 }
 
 static void __exit daedalus_exit(void)
 {
 	platform_driver_unregister(&daedalus_platform_driver);
 	platform_device_unregister(daedalus_platform_device);
+	daedalus_chardev_exit();
 }
 
 module_init(daedalus_init);