Structured parse diagnostics on ProtocolError

[EONRaider]

Target: 2.0.0 (Tier 2 — from codec to platform)

Strictness is the library's security story: we raise where scapy
silently fills defaults. But every exception carries only a formatted
string. Verified — the full attribute set on a raised
TruncatedHeaderError is args, add_note, with_traceback. There is
no offset, no field, no protocol, no expected/actual.

Ethernet TruncatedHeaderError "Ethernet header needs 14 bytes, buffer holds 4"
IPv4 InvalidFieldError "IPv4 IHL must be at least 5, got 0"

Everything useful is in the prose. A fuzzing harness, a conformance
suite or a protocol-validation tool that wants to know where a parse
failed has to regex our error messages.

Why this is a differentiator, not just a nicety

Scapy mostly does not raise at all. dpkt raises bare UnpackError /
NeedData with nothing attached. For the audience we most want —
people parsing untrusted input — this is the difference between a
library that rejects bad input and one that explains it. No
competitor offers it.

What to do

Add structured attributes to ProtocolError and populate them at every
raise site. Candidates: offset (byte offset into the buffer handed to
decode), protocol (the class), field (name), expected,
actual.

Design questions:

• Offsets are per-decode()-call, since each layer receives its own
  buffer slice. An offset absolute to the original frame needs the walker
  (Ship decode_frame() — the chain walker belongs in the library #88) to add its cursor. Decide whether the exception carries a
  relative offset that decode_frame rebases, or whether both are
  exposed.
• Keep the message strings as they are — they are good, and tests
  assert on some of them.

Acceptance criteria

• Every ProtocolError raised from src/ carries at least
  protocol and, where meaningful, offset and field.
• Existing message text is unchanged.
• decode_frame rebases relative offsets to the whole frame, or
  documents why it does not.