Add privilege dropping, AF_PACKET demo, ICMP, checksum validation, --help, and TCP reassembly

Rounds out the build order in PLAN.md with six incremental additions:
drop root privileges immediately after opening the capture handle;
a standalone AF_PACKET/mmap ring-buffer demo (kept separate from
CaptureSession, see its header comment for why); ICMPv4/ICMPv6 type
and code decoding; opt-in IPv4/TCP/UDP checksum validation (-c);
CLI --help; and opt-in, in-order-only TCP stream reassembly (-a) so
HTTP requests/responses split across segments can be seen whole.
Each addition is unit-tested and, where it touches live traffic
behavior, verified against real captured packets - see PLAN.md's
Decisions section for the verification notes on each.

1 files changed, 18 insertions, 1 deletions

diff --git a/include/wireframe/summarize.hpp b/include/wireframe/summarize.hpp
index 59aa621..e7e9ae3 100644
--- a/include/wireframe/summarize.hpp
+++ b/include/wireframe/summarize.hpp
@@ -13,6 +13,7 @@
 #include "wireframe/l7/http.hpp"
 #include "wireframe/l7/tls.hpp"
 #include "wireframe/net/ethernet.hpp"
+#include "wireframe/net/icmp.hpp"
 #include "wireframe/net/ipv4.hpp"
 #include "wireframe/net/ipv6.hpp"
 #include "wireframe/net/tcp.hpp"
@@ -122,8 +123,24 @@ inline std::string summarize_transport_and_above(const IpInfo& info) {
                 out += " | " + *l7;
             }
         }
+    } else if (info.proto == net::kProtoIcmp) {
+        if (auto icmp = net::parse_icmpv4(info.payload)) {
+            out += " | ICMP " + net::icmpv4_type_name(icmp->type);
+            if (icmp->identifier) {
+                out += " id=" + std::to_string(*icmp->identifier) +
+                       " seq=" + std::to_string(*icmp->sequence);
+            }
+        }
     } else if (info.proto == net::kNextHeaderIcmpv6) {
-        out += " | ICMPv6";
+        if (auto icmp = net::parse_icmpv6(info.payload)) {
+            out += " | ICMPv6 " + net::icmpv6_type_name(icmp->type);
+            if (icmp->identifier) {
+                out += " id=" + std::to_string(*icmp->identifier) +
+                       " seq=" + std::to_string(*icmp->sequence);
+            }
+        } else {
+            out += " | ICMPv6";  // truncated: at least say what it is
+        }
     }
     return out;
 }