Add deferred worksheet font color edits

[shps951023]

Adds a Rust-native fluent editor for deferred font-color updates to existing worksheet cells.

• collect typed CellReference/RgbColor operations and normalize them at save()
• apply last-write-wins semantics in worksheet coordinate order
• preserve unrelated font and XF properties while deduplicating generated styles
• stream worksheet XML through temporary storage and atomically validate/commit the package
• cover reverse ordering, duplicate updates, repeated saves, no-op saves, case-insensitive sheet lookup, style reuse, and rollback on failure
• document the Rust-only compatibility boundary in English and Simplified Chinese

Validation:

• cargo +1.85.0 test --workspace --all-targets --locked
• cargo +1.85.0 test -p miniexcel --test insert --locked
• cargo +1.85.0 clippy --workspace --all-targets --locked -- -D warnings
• cargo +1.85.0 doc --workspace --no-deps --locked
• focused rustfmt checks for changed Rust files

Closes #16

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