v0.5.7

Summary

• Add FORMAT-SPEC.md — canonical BrightDate % format characters across findutils, iputils, btime, and BSH.
• btime -f adds %dE, %dU, %dS milliday specifiers matching BSH default TIMEFMT (%.6f md).

Test plan

• cargo test -p btime
• btime -f 'real=%dE user=%dU sys=%dS' true