feat(bindings): scalar value -> span casts (date, timestamptz, int, bigint, double)

[estebanzimanyi]

Summary

Registers cast functions so DuckDB recognises:

From	To
INTEGER	INTSPAN
BIGINT	BIGINTSPAN
DOUBLE	FLOATSPAN
DATE	DATESPAN
TIMESTAMP_TZ	TSTZSPAN

The cast entry point Value_to_span_cast was previously declared in src/include/temporal/span_functions.hpp but never defined; defines it to delegate to Value_to_span_core (same path the existing span(value) scalar function uses).

Also fixes a pre-existing fall-through bug in Value_to_span_core: the T_INT4 case was missing break; and silently fell through to the T_INT8 branch, which would read the INT32 source vector as INT64 and trip DuckDB's internal Expected vector of type INT64, but found vector of type INT32 assertion. The bug only surfaced once anything actually exercised the INT4 branch via this entry point, which the new INTEGER -> INTSPAN cast does.

Smoke test

SELECT 5::intspan;                          -- [5, 6)
SELECT 5::BIGINT::bigintspan;               -- [5, 6)
SELECT 1.5::DOUBLE::floatspan;              -- [1.5, 1.5]
SELECT date '2000-01-01'::datespan;         -- [2000-01-01, 2000-01-02)
SELECT timestamptz '2000-01-01'::tstzspan;  -- [2000-01-01 00:00:00+00, ...]

Test plan

• make release then TZ=UTC ./build/release/test/unittest "<proj>/test/*" — full suite passes.

Activates the date / timestamptz cast skip block in test/sql/parity/003_span.test (open in PR #13) once both land.