test: property-based tests for basis-point conversion

[Hollujay]

Summary

Uptime is stored in basis points to avoid floats. The conversion from raw
metrics to basis points is arithmetic that is easy to get subtly wrong at the
edges — 0%, 100%, rounding at the half, and values that would overflow or
truncate.

Acceptance Criteria

• Add property-based tests (proptest or similar) over the conversion
• Explicitly cover 0, 10000 (100%), and rounding boundaries
• Assert the result is always within the valid basis-point range, for any
  input the parser can produce

Tech Stack

Rust, proptest