docs: add a glossary for terms with several meanings (registry, schema/data model/info, default engine) #1949

A few words in the docs carry several meanings, and no single page tells them apart, so a link to one explanation cannot resolve them:

• registry: a pcapkit.const enumeration (pcapkit/const/index.rst:14), an upstream IANA registry (index.rst:117), the registration functions (foundation/registry.rst), and EnumMeta.registry (pcapkit/protocols/schema/schema.py:1259). contributing/conventions/test-coverage.rst:11 uses two of these senses close together.
• schema / data model / info: defined only in the extension guide (ext.rst). Protocol.data is raw bytes, Protocol.info returns a data model but its docstring calls it an "Info dict" (protocol.py:607), and Protocol.schema is "Schema data".
• default engine: used across the engine and extraction docs, never defined. engines/builtin.rst does not use the word.

Proposal: add docs/source/glossary.rst, a .. glossary:: holding only these terms. Each entry gets one or two sentences saying which sense applies, plus a link to the page that explains it, so it never becomes a second explanation. Put it in the "Guides & Changelog" toctree between demo and changelog. Point the ambiguous mentions at it with :term:, and fix the "Info dict" docstring.

Terms that have one canonical explanation keep linking to it (:ref:/:doc: or an rst_epilog substitution, as in #1948), not to the glossary. A test pins the entries and that every :term: resolves.

Blocked until #1946 and #1948 merge: both edit protocol.py and the engines pages this would touch.