fix(kernel): TIMESTAMP range + DDL affected-rows parity with Thrift

[mani-mathur-arch]

What

Two SEA-via-kernel↔Thrift parity fixes surfaced by the Go comparator (two database/sql connections against the same warehouse, differing only in useKernel). Both are kernel-path-only divergences from the default Thrift path; the fix is entirely Go-side (no kernel/C-ABI change).

• TIMESTAMP out-of-nanosecond-range wrapping. The kernel Rows path converted arrow timestamps with arrow's ToTime, which forms an int64-nanosecond intermediate (value * unit-multiplier). For a microsecond TIMESTAMP — Databricks' wire unit — that overflows int64 outside ~1678–2262 and silently wraps a valid instant to a wrong one: TIMESTAMP '0001-01-01' scanned back as 1754-08-30, '9999-12-31' as 1816-03-30. Now converts via the unit-specific time.Unix/UnixMilli/UnixMicro constructors, which split into (seconds, nanoseconds) and represent the full 0001–9999 range. The default Thrift path is unaffected — it receives TIMESTAMP as a preformatted server string and never runs this multiply.

• DDL affected-rows reported as -1 instead of 0. The C ABI returns -1 for "not applicable / unknown" affected-row counts (DDL, SELECT, or a warehouse that doesn't surface the counter — the kernel's num_modified_rows Option<i64> None). The Thrift path reports 0 in those cases, so Result.RowsAffected() diverged: a CREATE/DROP SCHEMA returned -1 on the kernel backend vs 0 on Thrift. The -1 sentinel is now folded to 0; a real DML count (>= 0) passes through unchanged.

Testing

• TestScanCellTimestampOutOfNanoRange — pins both repro endpoints (0001-01-01, 9999-12-31) plus the int64-ns boundary years; the existing timestamp unit/location tests still pass.
• TestNormalizeAffectedRows — table test for the sentinel-normalization rule, factored into a pure (untagged) normalizeAffectedRows so it runs under the default CGO_ENABLED=0 build.
• Verified on both build paths: default pure-Go (CGO_ENABLED=0) and tagged cgo (-tags databricks_kernel, linking the pinned kernel .a) — build + unit tests green on each.

Stacked on mani/sea-kernel-new-items (#412).

Co-authored-by: Isaac