feat(bindings): extent(tnumber) → tbox + bundle extent overloads in one set

[estebanzimanyi]

Summary

Adds 2 new aggregate overloads:

Overload	Returns
`extent(tint)`	`tbox` (TBOXINT XT)
`extent(tfloat)`	`tbox` (TBOXFLOAT XT)

```sql
SELECT extent(t) FROM (VALUES (tint '[1@2000-01-01, 5@2000-01-05]'),
(tint '[10@2000-01-03, 20@2000-01-08]')) tt(t);
-- TBOXINT XT([1, 21),[2000-01-01 00:00:00+00, 2000-01-08 00:00:00+00])
```

Implementation

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

Component	What it does
`TboxExtentState`	Inline `TBox box` + `bool isset`. TBox is fixed-size POD (Span period + Span span + int16 flags).
`TnumberExtentFunction::Operation`	Copies input Temporal blob into a heap allocation, calls MEOS `tnumber_extent_transfn`. First call returns a palloc'd TBox which we memcpy-and-free into state; subsequent calls expand state in place.
`TnumberExtentFunction::Combine`	Merges two TBox states via `tbox_expand` — replicates the inline path in MEOS when both operands are non-null TBoxes.
`TnumberExtentFunction::Finalize`	Returns blob or NULL.

API refactor (driven by adding the second module)

Previously each module registered its own `AggregateFunctionSet("extent")` directly via the loader. That worked when only `SpanAggregates` existed (PR #47). Adding `TemporalAggregates` with the same set name surfaced a DuckDB constraint: a second registration with the same aggregate name triggers an ALTER path, but `CreateAggregateFunctionInfo` doesn't override `GetAlterInfo()`, so it throws `NotImplementedException`.

Fix: both modules now expose `AddExtentOverloads(AggregateFunctionSet &)` that mutate a shared set. The load site in `LoadInternal` builds one set, calls both, then registers exactly once:

```cpp
AggregateFunctionSet extent_set("extent");
SpanAggregates::AddExtentOverloads(extent_set);
TemporalAggregates::AddExtentOverloads(extent_set);
loader.RegisterFunction(std::move(extent_set));
```

Stacked on

• feat(bindings): aggregate function infrastructure — extent(span) for all 5 span types #47 (AggregateFunction infrastructure + extent(span))
• feat(bindings): extend extent(*) to spanset and set surfaces #48 (extent(spanset/set))

Test plan

• `extent(tint)`, `extent(tfloat)` produce correct TBOXINT/TBOXFLOAT
• NULL handling
• Regression: existing span/spanset/set overloads from feat(bindings): aggregate function infrastructure — extent(span) for all 5 span types #47/feat(bindings): extend extent(*) to spanset and set surfaces #48 still work after the bundling refactor
• Existing test suite still passes

[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.