feat(resolvers,wire): pluggable resolvers + wire-layer codec (step 4)

[estebanzimanyi]

Stacks on #6 (catalog-driven dispatcher foundation). Adds the two layers a production endpoint migration needs alongside the dispatcher:

What ships

File	Role
mobilityapi/resolvers.py	stub_resolver(registry) / pymeos_resolver() / default_resolver(prefer_pymeos=True) — pick the MEOS function implementation. PyMEOS is lazy-imported on first call so MobilityAPI can be installed without it.
mobilityapi/wire.py	WireCodec keyed by encoding name (mfjson / text / wkb / hexwkb) — decode HTTP wire values to PyMEOS objects, encode PyMEOS results back. stub_codec for tests; pymeos_codec() for production.
mobilityapi/__init__.py	Re-exports Dispatcher, FunctionSignature, stub_resolver, pymeos_resolver, default_resolver, WireCodec, stub_codec, pymeos_codec, and the ENCODING_* constants. Endpoint migrations import from one module.
tests/test_resolvers.py	7 tests covering: stub registry hit/miss, pymeos_resolver lazy-import contract, default_resolver fallback path, prefer_pymeos=False short-circuit.
tests/test_wire.py	8 tests covering: encoding constants, stub codec round-trip, missing-encoding error, has_decoder/has_encoder, pymeos_codec lazy-import contract.
.github/workflows/python.yml	Pytest job extended to run all three test files together (the existing 12 dispatcher tests + 7 resolver tests + 8 wire tests = 27 unit tests).

All 15 new tests pass locally (python3 -m pytest tests/test_resolvers.py tests/test_wire.py -v).

Migration plan unlocked

With #6 + this PR in place, each of the 5 endpoint migrations becomes the same ~50-100 LoC pattern:

from mobilityapi import Dispatcher, default_resolver, pymeos_codec, ENCODING_MFJSON

_DISPATCHER = Dispatcher(resolver=default_resolver())
_CODEC      = pymeos_codec()

def get_velocity(self, collection_id, feature_id, geometry_id, connection, cursor):
    # 1. fetch the trajectory from the DB
    cursor.execute("SELECT asMfjson(trajectory) FROM temporal_geometries WHERE id = %s", (geometry_id,))
    traj_mfjson = cursor.fetchone()[0]

    # 2. dispatch to MEOS via PyMEOS
    traj  = _CODEC.decode(ENCODING_MFJSON, traj_mfjson)
    speed = _DISPATCHER.dispatch("tpoint_speed", {"temp": traj})

    # 3. encode back to MF-JSON for the HTTP response
    return send_json_response(self, _CODEC.encode(ENCODING_MFJSON, speed))

Production wiring

Production runs install PyMEOS via requirements.txt (not added in this PR — the migration PR that flips the first endpoint adds it). Until then, default_resolver() returns a stub that raises NotImplementedError on first dispatch with an actionable message, so MobilityAPI can be installed and unit-tested without PyMEOS.

Dependency chain

Stacks on	Status
#6 (dispatcher foundation)	OPEN, green (12 dispatcher tests in CI)

Next stacked PRs

Step	Scope
#N+1	Add pymeos>=1.4 to requirements.txt; flip the first endpoint to use the dispatcher (recommended: temporal_geom_query/velocity.py → Dispatcher.dispatch("tpoint_speed", …)).
#N+2	Remaining 4 endpoint migrations (acceleration.py, distance.py, temporal_geom_seq/, temporal_properties/).
#N+3 (step 5 of ingestion plan)	Swap the dispatcher routes for the OGC API – Moving Features paths exposed by MEOS-API #13.