fix(reassembly): an IP datagram over 65,535 octets can read COMPLETE but short, or vanish in strict mode #1566

An IP datagram longer than 65,535 octets can be reported COMPLETE with its tail missing. In strict mode, a fragment clipped at the buffer edge can make the datagram disappear entirely. These were found in the #1564 review and re-measured on its head, e7c9e5fba. On main, the same inputs raise IndexError instead (#1565).

COMPLETE but short.

• Input: fragments covering octets 0–65527, then a malformed 7-octet MF=1 fragment at offset 65528 (TL 27), then a final fragment at offset 64 with TL 65535.
• The total data length (TDL) is 65,579, yet the datagram reads COMPLETE with 65,535 octets, in both strict and lax mode.
• Cause: both completion checks test all(RCVBT[0:(TDL + 7) // 8]), and that slice silently stops at RCVBT's 8,192 entries (pcapkit/foundation/reassembly/ip.py:230-232 and :334-336 on e7c9e5fba).

Silent drop in strict mode.

• Input: an 8-octet MF=1 fragment at offset 65528 (TL 28) is clipped to 7 octets by the buffer (:185).
• The check held != length then sets no RCVBT bit (:215). Strict mode returns no datagram at all, and lax mode returns an empty PARTIAL one.
• A 7-octet fragment at the same offset is reported. One extra octet changes the outcome.

Expected: a datagram that cannot fit is never COMPLETE, and a clipped fragment counts consistently. The reviewer's candidate fix is to require stop <= len(RCVBT) in both completion checks. In a scratch tree, that turns the first repro into PARTIAL while a legitimate 65,523-octet datagram still completes.

Blocked until #1564 merges, because the fix is in the same file.