Represent mobility observables explicitly and degrade searches safely

[jspaezp]

Problem

The arena has one mobility: Vec<f32> column and one mutually exclusive MobilityKind, but a library can contain any combination of:

• inverse reduced mobility (1/K0)
• drift time
• FAIMS compensation voltage
• collision cross section (CCS; preserved metadata, not necessarily a search axis)

The mzSpecLib reader currently stores 1/K0 or fallback drift time in the same column, maps absence to 0.0, and leaves the library reporting Ook0. Consequently im_axis_filterable can apply a TIMS window around a fake zero and silently produce no IDs.

Required invariants

• Preserve each observable independently, including multiple observables on one entry.
• Represent absence explicitly; 0.0 is a valid value, not a sentinel.
• Derive capabilities from presence, not values.
• Restrict search only on compatible observables shared by the run and library entry. Otherwise leave that dimension unrestricted and its score unavailable/neutral.
• Never interpret drift time or FAIMS CV as 1/K0. Any conversion must be explicit and carry its calibration/physical assumptions.

This likely means renaming the current column to inverse_reduced_mobility and adding optional drift-time, FAIMS-CV, and CCS columns. Exact storage and capability representation are implementation choices.

Acceptance

• no-IM library + TIMS data degrades to search without IM restriction
• 1/K0 library + non-IM data does likewise
• matching 1/K0 inputs retain current filtering
• observables round-trip independently; entries carrying both FAIMS CV and 1/K0 retain both
• missing and legitimate-zero values remain distinguishable

Separate from mzSpecLib interchange because this changes the arena schema, capability/query contract, scoring availability, and serialization—not just parsing.