feat: crate-init — module skeleton (params, wire, producer, consumer, error)

- src/{params,wire,producer,consumer,error}.rs skeletons with module
  docs per overview.md §Module Map; lib.rs crate docs + re-exports
- constants in params.rs (OP_TUNNEL_OPEN, TUNNEL_ALPN,
  TUNNEL_OPEN_SCOPE) + establishment_reason helper (delegates to
  alkcall 0.7.0's ChannelOpenError::establishment_reason)
- MAX_DATAGRAM_LEN (65535) in wire.rs (ADR-003's frame-time bound)
- async-trait dropped: skeleton confirms no trait (ADR-004)

Verified: cargo check, clippy --all-targets -D warnings (native +
wasm32), fmt --check, test, wasm32 check — all clean

src/error.rs

@@ -0,0 +1,23 @@
+//! Tunnel error surfaces: the crate's `TunnelError` plus the typed
+//! open-error helpers consumers branch on (ADR-049 §4 surface).
+//!
+//! Skeleton module — filled by `tunnels/params` (codec + establishment
+//! error mapping) and `tunnels/consumer-session` (session errors).
+
+/// The crate-level error type (thiserror; AGENTS.md convention 2).
+///
+/// Skeleton: the codec and session variants land with
+/// `tunnels/wire-codec` and `tunnels/consumer-session`.
+#[derive(Debug, thiserror::Error)]
+pub enum TunnelError {
+    /// The data-plane codec rejected an operation (UDP framing).
+    #[error("codec: {0}")]
+    Codec(String),
+    /// A substrate-shaped operation was attempted on the wrong
+    /// data plane (e.g. `send_datagram` on a stream tunnel).
+    #[error("wrong substrate for this operation")]
+    WrongSubstrate,
+    /// An I/O failure on the session's data plane.
+    #[error("io: {0}")]
+    Io(#[from] std::io::Error),
+}