buffa-types: add jiff feature (Timestamp/Duration conversions)

[macalinao]

Summary

Adds a jiff Cargo feature to buffa-types providing conversions between the protobuf well-known types and jiff, mirroring the existing chrono feature:

• Timestamp ↔ jiff::Timestamp — reuses the shared TimestampError (InvalidNanos + Overflow). jiff::Timestamp reports sub-second nanos sign-consistently (negative pre-epoch), so the From impl re-normalizes to proto's always-non-negative nanos convention by borrowing a second.
• Duration ↔ jiff::SignedDuration — SignedDuration is the faithful analog of proto Duration (sign-consistent seconds + sub-second nanos; jiff::Span is deliberately not a target since its calendar units have no fixed length). Introduces a narrow DurationJiffError with only InvalidNanos: a validated proto Duration can never overflow SignedDuration.

Details

• no_std-compatible: jiff is pulled with default-features = false + alloc; buffa-types' std feature layers jiff/std on top via jiff?/std.
• Wired into buffa-test so the conversions are exercised by cargo test --workspace in CI (parity with chrono).
• jiff 0.2 declares MSRV 1.70, within buffa's 1.75 pin (so msrv-check stays green).
• Thorough unit + doc tests: pre-epoch borrow, exact-second pre-epoch, nanos boundaries, both jiff::Timestamp extremes, both SignedDuration extremes, i32::MIN nanos, bidirectional sign-mismatch rejection.

Checks

• cargo test -p buffa-types --features jiff ✅
• cargo clippy -p buffa-types --features jiff ✅ (clean)
• cargo check -p buffa-types --no-default-features --features jiff ✅ (no_std)
• cargo fmt --check ✅
• Changelog fragment added under .changes/unreleased/.

Reviewed pre-submission by the repo's rust-code-reviewer (no Critical/High/Medium correctness findings) and rust-api-ergonomics-reviewer (the flagged missing "proto JSON spec range" doc warning on the infallible From impls has been added for chrono parity).

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}