[Bug][Windows]: Require numeric schemaVersion 1 when loading themes

[rwang23]

Parent

Related to #63 (closed). This issue deliberately tracks only the Windows
runtime enforcement and does not claim to complete the broader cross-platform
theme-contract work.

What to build

Require a numeric schemaVersion: 1 when Windows loads a theme, preserve that
normalized value, and ensure a newly generated default custom theme satisfies
the same runtime contract.

Acceptance criteria

• Missing, string, and unsupported future schema-version values are rejected.
• Valid numeric schemaVersion: 1 is preserved through normalization.
• The default Windows custom-theme writer emits schemaVersion: 1.
• Writer-to-runtime payload validation and schema compatibility tests pass.

Blocked by

None.

Implementation

Draft implementation PR: #288. 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.