Summary

Replace the optional chrono timestamp support with an optional jiff feature, matching the sync zip crate's Jiff-facing API direction.

When jiff is enabled, new ZipEntry values now default their last-modified timestamp to the current UTC time via Jiff. When it is disabled, writes keep using the deterministic MS-DOS epoch timestamp of 1980-01-01 00:00:00.

This keeps ZipDateTime::default() deterministic while adding ZipDateTime::default_for_write() for feature-sensitive write defaults.

Changes

• Replace the chrono feature/dependency with optional jiff.
• Add ZipDateTime::{as_jiff, from_jiff} and Jiff conversion impls behind jiff.
• Use ZipDateTime::default_for_write() when constructing new entries.
• Keep the dev zip dependency from enabling its default timestamp/compression feature set.
• Add coverage for deterministic defaults without Jiff and current-time defaults with Jiff.

Validation

• cargo fmt --all --check
• cargo test --lib default_
• cargo test --lib --features jiff default_
• cargo test --lib
• cargo test --lib --all-features
• cargo check --no-default-features --features full-wasm --lib
• cargo clippy --lib -- -D warnings
• cargo clippy --lib --all-features -- -D warnings