feat(bindings): aggregate function infrastructure — extent(span) for all 5 span types

[estebanzimanyi]

Summary

Lands the first MobilityDuck AggregateFunction registration. Adds `extent(span)` that aggregates a column of spans into the bounding span over all values, for the 5 span types: `intspan`, `bigintspan`, `floatspan`, `datespan`, `tstzspan`.

```sql
SELECT extent(s) FROM (VALUES (intspan '[1, 5)'), (intspan '[3, 8)'), (intspan '[10, 12)')) t(s);
-- [1, 12)

SELECT extent(s) FROM (VALUES (datespan '[2000-01-01, 2000-01-05]'), (datespan '[2000-01-03, 2000-01-10]')) t(s);
-- [2000-01-01, 2000-01-11)
```

Implementation pattern

New module: `src/temporal/span_aggregates.cpp` + `src/include/temporal/span_aggregates.hpp`.

Component	What it does
`SpanExtentState`	Inline `Span span` + `bool isset`. MEOS Span is POD/fixed-size — state is trivially serialisable.
`SpanExtentFunction::Initialize`	Sets `isset = false`.
`SpanExtentFunction::Operation`	Decodes the input blob, calls MEOS `span_extent_transfn` to expand state in place.
`SpanExtentFunction::Combine`	Calls `span_extent_transfn(&target.span, &source.span)`.
`SpanExtentFunction::Finalize`	Returns `finalize_data.ReturnString(string_t(&state.span, sizeof(Span)))` or NULL when no input was seen.
Registration	One `AggregateFunctionSet("extent")` with 5 type-overloads (one per span type). Bundled because separate `AggregateFunction` registrations triggered DuckDB's catalog-alter path which doesn't implement `GetAlterInfo` for `CreateAggregateFunctionInfo`.

The same scaffolding extends naturally to the rest of the aggregate surface (architectural blocker for parity tests #21, #40, parts of #25):

• `extent(spanset)` via `spanset_extent_transfn`
• `extent(set)` via `set_extent_transfn`
• `extent(tnumber) → tbox` via `tnumber_extent_transfn`
• `tcount` / `tand` / `tor` / `tmin` / `tmax` / `tsum` via skiplist-backed states (more state setup)
• `spanUnion(span) → spanset` via `spanset_union_transfn`

This PR establishes the bridging pattern; follow-up PRs land each of the above on the same scaffolding.

Test plan

• Smoke test all 5 span types — produces correct extent
• NULL handling: mixed NULL + value → returns the value's extent; all-NULL → returns NULL
• Multi-row aggregation produces correct expanding bounds
• Existing test suite still passes locally