Remove blanket #pragma warning disable from Datetime.cs

[antonsynd]

Problem

src/Sharpy.Core/Datetime/Datetime.cs has three file-wide pragma suppressions at lines 1-3:

• CS8625 — "Cannot convert null literal to non-nullable reference type"
• CS8603 — "Possible null reference return"
• CS1591 — "Missing XML comment"

These defeat the solution-wide TreatWarningsAsErrors protection across the entire ~800-line file. The root cause is that Timezone parameters are typed as Timezone (non-nullable reference) but accept null defaults, and null comparisons are inconsistent (== null vs is null).

Proposed Fix

1. Annotate Timezone? nullable properly on all parameters that accept null
2. Fix inconsistent null handling patterns
3. Remove blanket CS8625/CS8603 suppressions
4. Add XML docs to public members missing them, then remove CS1591 suppression

Context

Found during implementation verification of plan-ee0da3 (Stdlib & Tooling Completion Post-Str Migration).

Acceptance Criteria

• No #pragma warning disable in Datetime.cs
• All datetime unit tests still pass
• No new compiler warnings

[antonsynd]

Closed — implemented in 820c076 (fix(core): replace Datetime.cs blanket pragmas with proper nullability). Three file-wide #pragma warning disable directives (CS8625, CS8603, CS1591) removed. Timezone parameters accepting null defaults marked Timezone?, all == null comparisons converted to is null, XML doc summaries added to public operators/overrides. Builds clean with TreatWarningsAsErrors.