Add deferred worksheet font-color editing #16

Goal

Add a Rust-native fluent API for deferred worksheet style updates:

• collect font-color changes before writing
• normalize cell order and apply last-write-wins at save time
• preserve unrelated cell style properties
• stream worksheet XML through temporary storage
• atomically validate and commit the rewritten workbook

Initial scope

• existing XLSX paths
• one worksheet per editor
• existing cells only
• RGB font colors
• Rust implementation only; no .NET adapter yet

Tests

Cover reverse-order updates, duplicate cell updates, style deduplication, no-op saves, case-insensitive sheet lookup, and rollback on failure.