refactor: self-registering packet registry + Position composition (non-breaking)

[hemna]

Summary

Two refactors to aprsd/packets/core.py on the experimental branch, inspired by the "type object" design pattern discussion:

1. Self-registering packet registry (72541bf)
2. Position value object composition (f30f764) — with a backward-compatibility shim so external plugins/extensions need no changes

Behavior is unchanged for existing callers: full test suite (782) passes, ruff format/lint clean, and all plugins/extensions in the ecosystem run their original, unmodified code (verified against all 26 plugin repos).

1. Self-registering packet registry

Replaces the get_packet_type() if/elif chain and the static TYPE_LOOKUP dict with a registry that packet classes register into automatically via __init_subclass__. Each concrete packet class declares a packet_type: ClassVar[str] and a matches(raw) classifier; TYPE_LOOKUP is derived from the registry so the two can no longer drift out of sync.

• factory() now returns Packet | UnknownPacket (was wrongly annotated -> type[Packet]); _type deserialization uses a registry-backed name lookup instead of globals()
• ThirdPartyPacket.subpacket is typed Optional[Packet] and from_dict() re-dispatches the raw sub-packet through factory() so dataclasses-json doesn't pre-build a lossy base Packet
• The beacon fallback (unrecognized format + latitude → beacon) is preserved in get_packet_type(); dead factory() branches removed

Adding a new packet type is now a single-class change — no central dispatch table to edit.

2. Position value object composition (non-breaking)

Collapses the Packet → GPSPacket → {Beacon, MicE, Telemetry, Object, Weather} data spine. All position data (lat/lon/altitude/symbol/comment/course/speed/PHG/DAO) and its conversion/formatting logic (zulu time, DDM, human_info) now lives in a frozen Position value object that GPSPacket composes (has-a instead of is-a).

Backward compatibility — external plugins/extensions keep working unmodified:

• GPS packet types accept both position=Position(...) (canonical) and the legacy flat kwargs (latitude=, longitude=, symbol=, comment=, speed=, ...). The legacy form emits a DeprecationWarning and is scheduled for removal in a future release.
• Flat attribute reads (packet.latitude, packet.comment, ...) keep working via read-only delegating properties.
• Per-type symbol defaults are preserved (ObjectPacket → r, WeatherPacket → _) when using the legacy flat form.
• On-disk JSON is unchanged: to_dict()/from_dict() flatten/unflatten the nested position, so persisted object-store files keep loading.
• The shim is explicitly marked TODO(remove) in the code (target: next major release) so the deprecated path doesn't linger.
• GPSPacket inherits DataClassJsonMixin directly because the @dataclass_json decorator unconditionally overwrites to_dict()/from_dict() and would clobber the flattening overrides.

Docs / changelog

• Added an [Unreleased] section to ChangeLog.md and regenerated docs/source/changelog.md
• docs/prepare_changelog.py now also normalizes the [Unreleased] heading level so the Sphinx copy stays consistent with version headings
• Packet API docs are autodoc-generated from the (updated) docstrings

Testing

• pytest tests → 782 passed
• ruff format + ruff check → clean
• Plugin compatibility verified against all 26 plugin/extension repos in ~/devel/mine/hamradio/aprsd-plugins: the 6 that use GPS packet constructors/reads pass with their original, unmodified code against this branch (repeat 60/60, webchat 71/73 — the 2 remaining failures are a pre-existing config issue that reproduces at the base commit without this PR, gps/weewx/rich-cli/digipi 1/1 each)