feat!: milliseconds on the axis, readouts on the recording, a date-only start, and the widened vocabularies

[aleskiontherun]

Implements the specification change in divejson/divejson#30, pinning SPEC_REF at its squash and re-vendoring schema/, fixtures/ and docs/ from it. The profile axis is milliseconds: every reader multiplies its source's seconds by a thousand before rounding (converter.milliseconds), so a fraction the source states keeps its place, and the UDDF writer writes <divetime> back as seconds with a fraction only where needed; .ssrf's M:SS clock, which takes a second factor on the way to the axis, is held to the representability bound as a product. A device's readouts — surface_pressure, cns_start, cns_end, otu_start, otu_end — and FIT's salinity setting sit on the recording its file produced; a figure a source states once for the dive (UDDF <surfacepressure>, Subsurface @cns/@otu) goes to the primary recording, reported resolved where the dive has two, and a readout alone makes a recording. A dive whose source states a date and no time of day starts on the bare date rather than midnight, the validator accepts that on a dive's started_at only, and the UDDF writer writes the bare <datetime> (the suite's XSD pass widens that one element's spelling for itself). po2_limit is ppo2_limit, notes is no longer cut, and divejson validate no longer checks member order.

Verified with pytest (1150 passed), divejson conform fixtures --strict (77 cases) and the pin's byte and ancestor checks against the specification's main; ruff and mypy unchanged from main.

🤖 Generated with Claude Code