Rust refactor: collapse 3 hand-rolled civil-date implementations onto the time crate

[willwashburn]

Context

The same Howard Hinnant civil-date algorithm is hand-rolled three times in three files:

1. crates/relayburn-sdk/src/ledger/writer.rs:26 now_iso — though the function name is misleading, see #(forthcoming bug-bundle issue).
2. crates/relayburn-sdk/src/ingest/pending_stamps.rs:477-489 format_iso_8601, :491-530 parse_iso_ms, :638+ civil_from_unix_seconds / unix_seconds_from_civil.
3. crates/relayburn-sdk/src/query_verbs.rs:117-142 format_iso_z / days_to_ymd.

Plus a fourth ~240-line copy in crates/relayburn-cli/src/commands/compare.rs:258-495 — covered separately as "switch compare.rs to SDK normalize_since".

Proposed fix

Add time = "0.3" to relayburn-sdk/Cargo.toml (zero-runtime, no feature gates needed for the formatting we use). Replace all three implementations with OffsetDateTime / Date operations. The functions become 1-2 liners.

Verify byte-identical output against the existing golden tests + the conformance gate. The TS sibling uses ISO-8601 / millisecond precision; time::format_description::well_known::Iso8601 matches.

Estimated removal: ~250 LOC.

Out of scope

• Whole chrono adoption — time is sufficient for what we need and has fewer transitive deps.
• Touching compare.rs — that's covered by the separate normalize_since migration issue.

References

• Ingest+ledger review notes from the May 2026 Rust review.