proto: bump PROTO_MAX_PAYLOAD 64 KiB -> 1 MiB (closes #19)

Real H.264 access units routinely exceed the previous 64 KiB cap
on the chardev wire protocol:

  720p worst-case I-frame  ~200 KiB
  1080p worst-case I-frame ~500 KiB

libva-v4l2-request-fourier detects the under-sized OUTPUT-MPLANE
buffer and tries to grow it via VIDIOC_S_FMT to 147456 B, but
daedalus_fill_output_fmt unconditionally pins sizeimage to
DAEDALUS_MAX_BITSTREAM (= 65484) regardless of userspace's
request.  Firefox loses the slice, falls back to libmozavcodec
SW for the rest of the session.

Bumping the wire-protocol cap to 1 MiB lifts the kernel
OUTPUT_MPLANE sizeimage with it (DAEDALUS_MAX_BITSTREAM is derived
from the same #define).  All allocations (kernel kmalloc /
kmemdup, daemon read buffer, vb2 plane backing) are dynamic and
sized per-payload at runtime, so the only growth is the daemon's
startup read buffer (one ~1 MiB allocation per daemon process)
and the V4L2 OUTPUT_MPLANE per-buffer size.  KMALLOC_MAX_SIZE on
aarch64 SLUB is several MiB; 1 MiB is well within bounds.  Other
V4L2 stateless decoders (cedrus, rkvdec, hantro) report 1-4 MiB
OUTPUT_MPLANE sizeimage — this puts daedalus at the conservative
end of normal.

## Compatibility

#define-only change; struct layout unchanged.  But the
effective cap is the smaller of (kernel cap, daemon cap), so:
- new daemon + stale kernel: still capped at 64 KiB until the
  kernel module rebuilds.
- new kernel + stale daemon: same.
Lock-step install of daedalus-v4l2 + daedalus-v4l2-dkms is
therefore required for the fix to take effect; mirrors the
PR-#7/#8 cadence.

## NOT changed in this commit

- daedalus_fill_output_fmt still hardcodes sizeimage =
  DAEDALUS_MAX_BITSTREAM regardless of userspace request.
  Acceptable: vb2 will allocate up to that, and libva's resize-
  test now sees the kernel report a sizeimage at-least-as-large
  as what it asked for (147456 < 1048524).  A future cleanup
  could respect userspace's S_FMT.sizeimage clamped to the cap,
  to save memory on tiny streams.
- chardev kmalloc → kvmalloc swap (only matters above
  KMALLOC_MAX_SIZE, not here).

Refs #19.

include/daedalus_v4l2_proto.h

@@ -71,7 +71,18 @@ struct daedalus_msg_hdr {
 	__u32 reserved;
 };
 
-#define DAEDALUS_PROTO_MAX_PAYLOAD	(64u * 1024u)	/* 64 KiB */
+/*
+ * Wire-protocol payload cap.  Sized to comfortably hold real-world
+ * H.264 / VP9 / AV1 access-unit bitstreams:
+ *   - 720p H.264 worst-case I-frame:  ~200 KiB
+ *   - 1080p H.264 worst-case I-frame: ~500 KiB
+ *   - 4K H.264 worst-case I-frame:    ~2 MiB (would need a bump)
+ * 1 MiB is the conservative end of what cedrus / rkvdec / hantro
+ * report as OUTPUT_MPLANE sizeimage.  Allocations (chardev kmalloc
+ * / kmemdup, daemon read buffer, vb2 plane backing) are sized per-
+ * payload at runtime; this only sets the ceiling.  Issue #19.
+ */
+#define DAEDALUS_PROTO_MAX_PAYLOAD	(1024u * 1024u)	/* 1 MiB */
 
 /* -- REQ_DECODE / RESP_FRAME payload structures ---------------------- */