fix(pptx): preserve package identity when removing slides

[RainLib]

What changed

• Resolve logical slide indexes through presentation relationships instead of assuming slideN.xml.
• Remove the slide ID, presentation relationship, slide part, slide rels, content type override, and dangling custom-show references.
• Allocate unused slide part, slide ID, and relationship ID when adding after a middle-page deletion.
• Record the C# parity baseline and migration ledger under docs/csharp-parity.

Why

Deleting a middle slide could leave dangling package references, and later operations could edit or overwrite the wrong physical slide part.

Validation

• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --workspace
• Package-level custom-show cleanup tests
• CLI smoke: create → add → remove middle → edit logical slide → add → validate