fix(hip): three BitField namespaces leave reserved bits unnamed, so they rebuild as zero #1417

Describe the bug
Three HIP BitField namespaces leave bits unnamed, and the unnamed bits are packed as zero on rebuild. #1416's invariant test found them (start, length):

• HIP.ver: (4, 3)
• RouteDstParameter.flags: (2, 14)
• RouteViaParameter.flags: (2, 14)

To Reproduce
Set those bits non-zero in a raw HIP packet, parse it, and rebuild with HIP.from_data(p.info).data. The bits come back as zero.

Expected behavior
Byte-exact, per the every-round-trip ruling. Name a reserved sub-field at each site (the #654 policy) and carry it through the reader, data model and make(), as #1253 did. Then delete the three entries from KNOWN_GAPS in tests/protocols/schema/test_bitfield_coverage_unit.py.

Part of #1288.