Fix IPv4/IPv6 fragment continuation data being decoded as fake transport headers

A real correctness bug, not just missing visibility: a non-first
fragment's payload is pure continuation data with no TCP/UDP/ICMP
header in it at all, but it was being handed to the transport parsers
unconditionally, which could misread arbitrary payload bytes as port
numbers, sequence numbers, etc. and print a plausible-looking but
entirely fake decode.

IPv4 gained identification/more_fragments/fragment_offset fields; a
nonzero offset now stops summarize_packet before transport dispatch,
reporting the fragment instead. IPv6 expresses fragmentation as an
extension header instead, so the fix lives in
walk_ipv6_extension_headers(): it already walked past Fragment
headers, but never checked the offset before continuing on as if a
transport header followed - the same bug, reached through a
different path. Fixed with an ESP-style hard stop on a nonzero offset.

Caught and fixed a second bug while writing the first fix, before it
ever ran: the fragment's Identification field was a loop-local
variable, discarded the moment the walk continued past a *first*
fragment (offset zero) to keep decoding the real payload underneath --
every later return reported no fragment id even though one applied.
Fixed by hoisting it to a variable that persists across iterations,
the same category of mistake as an earlier IGMPv3 bug.

Fuzzed afterward regardless (fuzz_ipv4, fuzz_ipv6, fuzz_summarize,
~16.8M combined runs) - clean. Live-verified with a real 4000-byte
ping to the local gateway over the actual 1500-MTU interface: both
directions fragmented into 3 pieces each, the first decoded normally
with a fragmentation note, and the continuation fragments correctly
showed only fragment metadata, no fake ICMP decode attempted.

1 files changed, 42 insertions, 0 deletions

diff --git a/PLAN.md b/PLAN.md
index 7ad2649..64b49e3 100644
--- a/PLAN.md
+++ b/PLAN.md
@@ -685,3 +685,45 @@ None currently open.
   real intermediate routers - this machine's own gateway, then real
   ISP infrastructure several hops out - all correctly showing "[this
   machine -> 8.8.8.8 proto=1]", matching traceroute's own hop output.
+- IPv4/IPv6 fragmentation: a real correctness fix, not just added
+  visibility. Before this, a non-first fragment's payload - pure
+  continuation data, no TCP/UDP/ICMP header present at all - was
+  handed to the transport parsers unconditionally, which could
+  misread arbitrary payload bytes as port numbers, sequence numbers,
+  etc. and print a plausible-looking but entirely fake decode. IPv4
+  gained identification/more_fragments/fragment_offset fields on
+  Ipv4Header; a nonzero fragment_offset now stops summarize_packet
+  before it ever reaches transport dispatch, reporting the fragment
+  itself instead. IPv6 expresses fragmentation as a Fragment
+  extension header instead of header fields, so the fix lives in
+  walk_ipv6_extension_headers(): it already walked past Fragment
+  headers, but never checked the offset before continuing on to
+  decode whatever followed as if it were a transport header --
+  exactly the same bug, just reached through the extension-header
+  path instead of a header field. Fixed by having the walk stop
+  immediately (like the existing ESP hard-stop) whenever the offset is
+  nonzero, and reporting that stop via two new
+  Ipv6ExtensionWalkResult fields.
+  Caught and fixed a real bug in this fix while writing it, before it
+  ever ran: the first version computed a fragment's Identification
+  field as a loop-local variable, so it was silently discarded the
+  moment the walk continued past a *first* fragment's header (offset
+  zero) to keep decoding the real transport protocol underneath --
+  every return path after that point reported no fragment id at all,
+  even though one genuinely applied. Fixed by hoisting it to a
+  variable that persists across loop iterations, the same category of
+  mistake (and the same fix) as this session's earlier IGMPv3 bug:
+  state that needs to survive past the specific branch that computed
+  it. Fuzzed afterward regardless (fuzz_ipv4, fuzz_ipv6,
+  fuzz_summarize, ~16.8M combined runs) - clean, no crashes.
+  Live-verified with genuine IPv4 fragmentation: a 4000-byte ping to
+  the real local gateway over the actual 1500-MTU wlp1s0 interface,
+  captured on the wire. Both directions fragmented into exactly 3
+  pieces each; the first fragment decoded normally (ICMP Echo
+  Request/Reply, correct id/seq) with a "(fragmented, ...)" note, and
+  the two continuation fragments correctly showed only fragment
+  metadata - no fake ICMP decode attempted on their payload. IPv6
+  fragmentation is unit-tested (including the exact byte pattern that
+  triggered the fragment-id bug above) but not live-verified: this
+  machine has no real global IPv6 connectivity to generate genuine
+  IPv6-fragmented traffic against, only link-local addresses.