docs(api): context, IPX._next_exthdr_depth and pcapng schema helpers have no directive #2234

Is your feature request related to a problem? Please describe.
Members with their own docstrings render nowhere in the API docs:

• ProtocolBase.context: the Protocol autoclass in protocol.rst lists every other public property but not context, nor does the ProtocolBase group under Internal Definitions. The PCAPNG docstring's versionchanged and the 1.5.0 changelog entry for design(protocols): PCAPNG.context shadows ProtocolBase.context with a different type #2172 (refactor(pcapng)!: rename PCAPNG.context to section_context (#2172) #2205) both describe this property, and a reader has no page to land on.
• IPX._next_exthdr_depth: an override added for fix(ipx): the next layer is chosen by IPX Packet Type, looked up as an IP protocol number #2169 with its own docstring (it always returns 0), but the IPX autoclass in ipx.rst does not list it. Internet._next_exthdr_depth is documented in internet.rst, so this is a contract-level hook.
• pcapkit.protocols.schema.misc.pcapng: held_body is missing from Auxiliary Functions, where its sibling length callbacks (nonnegative, bounded_area, captured_area) are listed; FlagsField is missing beside OptionEnumField and OptionAreaField; PACKET_FLAGS is missing beside the autodata constants (BLOCK_MINIMUM and others).

Describe the solution you'd like
Add .. autoproperty:: context to the Protocol autoclass and context to the ProtocolBase py:property group. Add .. automethod:: _next_exthdr_depth to the IPX autoclass. In pcapng.rst, add autofunction:: ...held_body, autoclass:: ...FlagsField (:members:, :show-inheritance:) and autodata:: ...PACKET_FLAGS (:no-value:). Then link the bare ``context`` in the PCAPNG versionchanged to :attr:.

Describe alternatives you've considered
Leaving _next_exthdr_depth out as a private name. That goes against the house rule: an override of a documented must-implement hook stays documented.

Additional context
Checked against origin/main 91147a1. _held_octets and _secrets_cut in the same module are also undocumented; they are private helpers, so whether they get entries is a separate call.