Make partial refund unit rounding partition-safe

[prissca]

What changed

• persist an immutable refund basis for each position funding epoch
• calculate every partial refund from cumulative refunded principal against that basis
• reset the basis only when new funding changes the combined position
• explicitly reject refunds below the current unit granularity and clear basis state on terminal refunds
• add one-shot versus partitioned and dust-regression tests

Why

Independent per-call rounding could let call partitioning influence retained units. Cumulative refund accounting makes the final entitlement depend only on total refunded principal.

Validation

• Rust sources parse successfully with rustfmt
• contract tests could not link on this machine because the MSVC link.exe build tool is not installed

Closes #166

[prissca]

Thanks for the contribution! Merging this now as part of a maintainer PR sweep.