Fix proportional emergency withdrawals

[er1c-cartman]

Summary:

• Updates emergency withdrawal to refund the current escrowed round proportionally to members who actually contributed.
• Sends rounding remainders to a contributing member, and keeps the no-contribution fallback from leaving tokens stuck.
• Adds a regression test for two completed rounds followed by an incomplete third round with unequal contribution state.

Fixes #76.

Validation:

• cargo test --package sorosave
• cargo fmt --check
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Notes:

• Generated build/test artifacts were not committed.
• No secrets, tokens, or private runtime data are included.