xlstream-eval: adjust DATE() year 0-1899, guard negative and overflow

[cilladev]

Fixes: #144

Problem

DATE(year, month, day) passed the year argument directly to ExcelDate::from_ymd without Excel's year-adjustment rule. Years 0-1899 produced massive negative serial numbers instead of the correct 1900-adjusted dates. Negative years and years producing serials past 9999-12-31 were also not guarded.

Fix

• Add 1900 to the year when 0 <= year < 1900 (Excel's documented behavior).
• Return #NUM! for negative years (negative serials).
• Return #NUM! when the resulting serial exceeds EXCEL_MAX_DATE_SERIAL (2,958,465 = 9999-12-31).

Issue description correction

The issue lists DATE(24,6,15) → 45458 (2024-06-15). Excel actually returns 8933 (1924-06-15) because the rule is strictly year + 1900, not century-guessing. The fix matches Excel's actual behavior, not the issue's third expected value.

Changes

• crates/xlstream-core/src/lib.rs: add EXCEL_MAX_DATE_SERIAL constant (2,958,465).
• crates/xlstream-eval/src/builtins/date.rs: year 0-1899 adjustment, negative year #NUM! guard, overflow #NUM! guard.
• 8 unit tests: boundaries (y=0, y=99, y=1899, y=1900, y=2026), negative, overflow, max valid (9999-12-31).
• Conformance fixture with 12 Excel-populated cells covering adjustment, boundaries, #NUM! errors, and fractional years.

Test plan

• Regression unit tests added and pass
• Conformance fixture created (Excel-populated) and passes (12/12, 0 mismatches)
• cargo test -p xlstream-eval passes (no regressions)
• make check passes

Closes #144