feat(bindings): extent(tgeompoint) -> stbox aggregate

[estebanzimanyi]

Summary

Adds the spatial branch of `extent()` — aggregates a column of `tgeompoint` values into the spatiotemporal bounding box (`STBox`) over all values.

```sql
SELECT extent(t) FROM (VALUES (tgeompoint 'Point(1 1)@2000-01-01'),
(tgeompoint 'Point(5 5)@2000-01-05')) tt(t);
-- STBOX XT(((1,1),(5,5)),[2000-01-01 00:00:00+00, 2000-01-05 00:00:00+00])
```

Implementation

New module: `src/geo/spatial_aggregates.cpp` + `src/include/geo/spatial_aggregates.hpp`.

Component	What it does
`StboxExtentState`	Inline `STBox box` + `bool isset` — fixed-size POD struct (time period + x/y/z bounds + SRID + flags).
`TspatialExtentFunction::Operation`	Copies the input Temporal blob and calls MEOS `tspatial_extent_transfn`. First call returns a palloc'd STBox which gets memcpy'd into state and freed; subsequent calls expand state in place.
`Combine`	Merges two STBox states via `stbox_expand`.
`Finalize`	Returns the box blob or NULL.

`SpatialAggregates::AddExtentOverloads` is wired into the same `extent()` `AggregateFunctionSet` that already carries span / spanset / set / tnumber / scalar overloads.

Test plan

• `extent(tgeompoint)` produces correct STBOX XT for instants and sequences
• No segfault on simple input

[estebanzimanyi]

Consolidated into #60 (aggregate cluster squash). The full chain of aggregate work is now reviewable as a single PR; this branch's commits are preserved in #60's history. Closing to minimize the review queue.