refactor(core): migrate packet processing from pnet to smoltcp (#2456)

Replace pnet_packet parsing and mutation across gateway packet paths with the existing smoltcp wire APIs. Preserve length validation, fragmentation classification, TCP flags, and checksum behavior while removing the core pnet_packet feature dependency.
Reject stale non-initiator OSPF sync sessions: only initiator requests may create missing sessions, and a rejection clears the old initiator role only when the remote session generation is unchanged. This fixes an unbounded RPC storm caused by a delayed route sync recreating a session after both peers relinquished the initiator role, with regression tests for session creation and response reordering.

easytier-core/Cargo.toml

@@ -44,7 +44,6 @@ parking_lot = "0.12.1"
 percent-encoding = "2.3.1"
 petgraph = "0.8.1"
 pin-project-lite = "0.2.13"
-pnet_packet = { version = "0.35.0", optional = true }
 prefix-trie = { version = "0.7.0", features = ["cidr"] }
 prost = "0.14.3"
 prost-types = "0.14.3"
@@ -116,7 +115,6 @@ rich-config-errors = ["dep:ariadne"]
 tcp-hole-punch = []
 proxy-packet = [
     "wrapped-transport",
-    "dep:pnet_packet",
     "dep:smoltcp",
     "smoltcp/std",
     "smoltcp/proto-ipv4",