feat(bindings): tagg_min(ttext) / tagg_max(ttext) overloads

[estebanzimanyi]

Summary

Adds the ttext branch of the temporal min/max aggregates that PR #52 explicitly skipped ("ttext skipped — text merge needs text_cmp plumbing"). MEOS exports `text_cmp` as a public API, so the wrapper datum-merge functions are 2-liners:

```cpp
static Datum mduck_datum_min_text(Datum l, Datum r) {
return text_cmp((text*)l, (text*)r) < 0 ? l : r;
}
```

Each merge fn casts both Datums to `text*` (PG varlena), invokes `text_cmp`, and returns whichever Datum the comparison selects.

Coverage

Aggregate	Inputs	Output
`tagg_min(ttext)`	ttext	ttext — per-instant minimum (lexicographic)
`tagg_max(ttext)`	ttext	ttext — per-instant maximum (lexicographic)

```sql
SELECT tagg_min(t) FROM (VALUES (ttext '"hello"@2000-01-01'), (ttext '"alpha"@2000-01-01')) tt(t);
-- {"alpha"@2000-01-01 00:00:00+00}

SELECT tagg_max(t) FROM (VALUES (ttext '"hello"@2000-01-01'), (ttext '"zebra"@2000-01-01')) tt(t);
-- {"zebra"@2000-01-01 00:00:00+00}
```

Stacked on

• feat(bindings): aggregate function infrastructure — extent(span) for all 5 span types #47 → feat(bindings): extend extent(*) to spanset and set surfaces #48 → feat(bindings): extent(tnumber) → tbox + bundle extent overloads in one set #49 → feat(bindings): tCount(temporal) aggregate — first skiplist-backed aggregate #50 → feat(bindings): tAnd / tOr / tagg_min / tagg_max / tagg_sum aggregates #52 → feat(bindings): spanUnion(span | spanset) → spanset aggregate #53 (aggregate stack)

Test plan

• tagg_min(ttext) / tagg_max(ttext) produce correct lexicographic results
• Existing int/float overloads still work (regression check)

[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.