add payload_len field to wire protocol header

Ethernet pads frames to 60 bytes minimum; without an explicit length
field the receiver cannot distinguish real payload from zero padding
(e.g. a 6-byte DISCOVER becomes 46 bytes after padding, failing the
'payload must be empty' check).

Header is now 8 bytes: [ver:1][type:1][session_id:4][payload_len:2]
(both multi-byte fields big-endian). The receiver slices exactly
payload_len bytes and ignores trailing padding.

Updated PROTOCOL.md, Rust frame.rs, and C# Frame.cs.

PROTOCOL.md

@@ -11,7 +11,9 @@ discriminator; there is no magic number inside the payload.
 +---------------+---------------+-------------------------------+
 | version       | type          | session_id                    |
 +---------------+---------------+-------------------------------+
-| payload ...                                                   |
+| payload_len (big-endian)      | payload ...                   |
++-------------------------------+                               +
+|                                                               |
 +---------------------------------------------------------------+
 ```
 
@@ -19,15 +21,19 @@ discriminator; there is no magic number inside the payload.
 - **type** (u8): frame type, see table below.
 - **session_id** (u32, big-endian): `0` for non-session frames; the
   server-assigned ID for session-scoped frames.
-- **payload** (variable): type-dependent. No length field is carried — the
-  Ethernet frame length from the capture gives the payload extent.
+- **payload_len** (u16, big-endian): number of payload bytes that follow.
+  The receiver reads exactly this many bytes and ignores any trailing
+  Ethernet padding (frames under 60 bytes are zero-padded by the NIC to
+  meet the minimum Ethernet frame size).
+- **payload** (`payload_len` bytes): type-dependent.
 
 No CRC, no retransmit, no ordering at this layer. TCP reliability is handled by
 the endpoints' TCP stacks; UDP (reserved) will rely on application-level
 mechanisms.
 
-Maximum payload: 1500 (Ethernet MTU) − 14 (eth header) − 6 (our header) =
-**1480 bytes**. Larger payloads are not emitted in v1.
+Maximum payload: 1500 (Ethernet MTU) − 8 (our header) = **1492 bytes**. In
+practice we cap at **1480** to stay conservative. Larger payloads are not
+emitted in v1.
 
 ## Frame types