feat(bindings): tAvg(tnumber) and wavg(tnumber, INTERVAL) aggregates

[estebanzimanyi]

Summary

Adds the temporal-average aggregate:

Aggregate	Inputs	Output
`tAvg(tint)`	tint	tfloat
`tAvg(tfloat)`	tfloat	tfloat

```sql
SELECT tAvg(t) FROM (VALUES (tint '5@2000-01-01'), (tint '3@2000-01-01')) tt(t);
-- {4@2000-01-01 00:00:00+00} -- avg of 5 and 3
```

Implementation

State holds a `SkipList` of `{sum, count}` pairs (MEOS `double2` layout — `{ double a; double b; }`). `Operation` calls `tnumber_tavg_transfn`. `Combine` merges two states via `temporal_skiplist_splice` with `mduck_datum_sum_double2` as the merge function — a local replica of MEOS's internal `datum_sum_double2` since that symbol is not in the MEOS public install headers (the underlying struct layout is, fortunately, simple enough to replicate exactly). `Finalize` calls `tnumber_tavg_finalfn` which computes per-instant `sum/count` and frees the skiplist; `state.skiplist` is nulled after to keep the destructor from double-freeing.

Test plan

• Same-instant `tAvg(5, 3)` correctly produces `4`
• Distinct instants preserve their per-instant values
• `tAvg(tfloat)` works with non-integer values
• No segfault on cleanup (destructor pattern from existing skiplist aggregates)

[estebanzimanyi]

Added a follow-up commit for wavg(tnumber, INTERVAL) -> tfloat — companion windowed time-weighted average. Same machinery as tAvg but binary input (Temporal + Interval), uses tnumber_wavg_transfn for accumulation. Reuses mduck_datum_sum_double2 and tnumber_tavg_finalfn.

SELECT wavg(t, INTERVAL '1 day')
  FROM (VALUES (tint '5@2000-01-01'), (tint '3@2000-01-02')) tt(t);
-- {[5@..., 4@..., 3@...]}    -- avg of (5, 3) = 4 at the overlap day

[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.