Rust parity · slice 3/11 — ISO-8601 to epoch via jiff

[ropdias]

Part of #27 · parity v1, slice 3/11 · implemented in #46 (base rust-parity-v1).

Scope

• fmt.rs: iso8601_to_epoch using jiff::Timestamp (UTC RFC 3339 only, no tz database). Empty or invalid input degrades the same way bash does, with no panic.

Bash oracle

epoch_from_iso in tokenline.sh (GNU date -d / BSD date -j -f).

Done when

• cargo fmt --check, cargo clippy --all-targets -- -D warnings and cargo test are green
• Same epoch as bash for fractional-second and Z timestamps; invalid input returns "no value" without panicking
• feat: parse ISO-8601 -> epoch via jiff (parity v1 · slice 3/11) #46 is merged into rust-parity-v1 with a merge commit, not squash or rebase, so the stacked slices above stay valid

#46 targets rust-parity-v1, not the default branch, so closing keywords do not fire. Close this issue by hand when #46 merges, and tick the slice in the #27 checklist.