[Bug][Theme packages]: Reject impossible RFC 3339 manifest timestamps

[rwang23]

Parent

None — standalone audit-remediation item.

What to build

Reject impossible RFC 3339 calendar timestamps in manifest.createdAt before
accepting a theme package, while keeping the canonical runtime validator and
both generated platform copies synchronized.

Acceptance criteria

• Impossible dates and invalid clock or offset ranges are rejected before Date.parse() normalization can accept them.
• Valid leap days, supported fractional seconds, and valid timezone offsets remain accepted.
• Runtime, macOS, and Windows validator copies stay synchronized.
• Regression tests cover valid and invalid timestamp boundaries.

Blocked by

None.

Implementation

Draft implementation PR: #286. This issue was created after the audit so the
independently reviewable remediation has a dedicated tracking record. It should
close only when the linked PR merges.