refactor(core): split delta module helpers

[peters]

Purpose

• split surge_core::releases::delta into focused internal modules for format detection, archive delta handling, sparse delta serialization, tree walking, and filesystem apply helpers
• keep the public delta API stable at surge_core::releases::delta
• remove the delta entry from the maintainability baseline

Behavior Impact

• no intended user-visible behavior change
• archive patch formats, sparse file-op deltas, and public delta apply/decode entrypoints keep their current behavior

Test Evidence

• cargo check -p surge-core
• cargo test -p surge-core releases::delta
• cargo clippy -p surge-core --all-targets --all-features -- -D warnings -W clippy::pedantic
• ./scripts/check-maintainability.sh
• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic
• dotnet format dotnet/Surge.slnx --verify-no-changes
• dotnet test dotnet/Surge.slnx --configuration Release

Migration Notes

• none