cost model: estimate disk and object-store I/O operation counts #345

Follow-up to #323.

Scope

Extend the analytical resource vector beyond scan bytes to estimate disk/object-store I/O operation counts where a physical plan performs explicit storage reads or writes. Keep operation counts separate from bytes until a versioned deployment calibration combines them.

Acceptance criteria

• Define units, formulas, assumptions, and required evidence for supported storage operators.
• Count shared physical I/O once by physical identity.
• Reject missing, stale, invalid, or non-finite storage evidence with Unavailable or a typed error.
• Add hand-computed and monotonicity tests for request size, bytes, and operation count.
• Export operation counts and model/evidence provenance through structured DAG cost annotations.