feat(geography): register GEOGRAPHY LogicalType (skeleton; PR #168 design)

[estebanzimanyi]

Foundation for the DuckDB GEOGRAPHY boundary documented in #168. Registers the LogicalType alias (BLOB carrying MEOS-WKB with the geodetic flag preserved in the type tag) so DuckDB columns can be declared as GEOGRAPHY.

What ships

File	Role
src/include/geo/geography.hpp	GeographyType struct declaration (LogicalType getter + RegisterType entry point), mirroring the shape of StboxType / TgeogpointType.
src/geo/geography.cpp	Implementation: BLOB with SetAlias("GEOGRAPHY") and loader.RegisterType("GEOGRAPHY", GEOGRAPHY()).
src/mobilityduck_extension.cpp	Load() wires GeographyType::RegisterType after StboxType.
CMakeLists.txt	Adds src/geo/geography.cpp to EXTENSION_SOURCES.
test/sql/geography.test	CREATE TABLE ... GEOGRAPHY / INSERT NULL / SELECT round-trip sanity test verifying the alias is registered.

Total: 4 new files + 2 edited files, ~50 LoC.

What it does NOT ship (follow-up PRs)

Item	LoC	Notes
I/O UDFs: ST_GeogFromText, ST_AsText, ST_AsBinary, ST_GeogFromBinary	~150	Thin shims over MEOS geog_in / geo_as_ewkt / geo_as_ewkb / geo_from_ewkb
Casts: GEOMETRY ⇄ GEOGRAPHY, GEOGRAPHY ⇄ TGEOGPOINT extras	~80	TGEOGPOINT(GEOGRAPHY, TIMESTAMPTZ) constructor reuses the existing temporal-geographic flow
Operations: length, area, eIntersects, nearestApproachDistance on GEOGRAPHY columns	(none required at the binding layer — MEOS geog_* already returns the right types; the dispatcher PR plugs them in)
Tests: round-trip, cast-matrix, numeric checks against MEOS-on-Postgres ground truth	~200	Compares values byte-for-byte against the same query run on mobilitydb

Total target surface (from #168's design): ~430 LoC across the four follow-ups. Each follow-up is independently mergeable on top of this skeleton.

Dependency chain

Stacks on	Status
#168 — geography boundary design doc	OPEN, docs-only

Verification

The sanity test test/sql/geography.test runs in CI:

CREATE TABLE geography_sanity (g GEOGRAPHY);
INSERT INTO geography_sanity VALUES (NULL);
SELECT g FROM geography_sanity;  -- → NULL
DROP TABLE geography_sanity;

If CI is green, the LogicalType is registered and the alias parses; the boundary doc's design is buildable. If CI surfaces any C++ compile errors (the skeleton mirrors StboxType exactly, so this is unlikely), they will be amended into the same commit per the green-CI-same-commit gate.