node: establish dataplane/ and session/ concept homes (behavior-neutral)

Reorganize the node module tree by concept rather than by
message-handling verb, as the first step of the node runtime
decomposition. Pure relocation: no wire, config, metric, or log
semantics change; the lib test count is unchanged (1577 passed).

Moves (git mv, 100% rename similarity):
- handlers/{forwarding,rx_loop,connected_udp,dispatch,encrypted}.rs
  -> node/dataplane/ — the whole RX hot path (the select! run loop,
  transit/local forwarding, the link-message router, the RX decrypt
  path with responder K-bit cutover + roam writes, and connected-UDP
  fast-path activation) now lives in one home.
- node/session.rs -> node/session/mod.rs — establishes the session
  concept home for the data/state types. The message-behavior file
  handlers/session.rs stays put for now (folds in with the later FSP
  session step).

The IK/XX-divergent establishment files (handlers/{handshake,rekey,
timeout}.rs) and the deferred-home files (handlers/{mmp,lookup}.rs)
deliberately stay in handlers/, to move once rather than twice.

Every module is reached through impl Node methods, so no call site or
re-export shim was needed. Updated in lockstep with the moves: the
module_path!-derived tracing targets in the two mesh-lab compose-trace
overlays, a structural test's include_str! source path, doc-comments
in proto/routing and the mesh-lab docs, and the stale source-location
citations (node/handlers/{forwarding,rx_loop,encrypted}.rs and
node/session.rs) in doc-comments and the discovery design doc.

src/peer/active.rs

@@ -1529,7 +1529,7 @@ mod tests {
     // === FMP rekey cutover: authenticate-before-promote ===
     //
     // IK-adapted analogue of the FSP trial-decrypt tests
-    // (node/session.rs `trial_decrypt_picks_pending_and_promotes` /
+    // (node/session/mod.rs `trial_decrypt_picks_pending_and_promotes` /
     // `trial_decrypt_failed_slot_leaves_replay_window_intact`). The FMP
     // cutover is gated on an authenticated decrypt against `pending`, not
     // the bare header K-bit. These tests exercise that primitive: