Fix #3: Prevent principal-sweeping on zero-side market resolution

[funds0033-cmyk]

Closes #3

Problem

When a resolver picked the empty (zero-participation) side of a one-sided market, the losing side's entire stake was swept into AccumulatedFees instead of being refunded — letting the resolver effectively steal the pool.

Changes

Resolving to an empty side is still allowed (one-sided markets stay resolvable), but that path can no longer take the pool:

• Provenance: empty-side principal now goes into a per-market ForfeitedPool, separate from global AccumulatedFees.
• Refunds: losers get their net stake back via the payout ledger; the protocol keeps only the standard 2% fee.
• Dispute window: 24h delay before claims or fee release. Admin can call freeze_market during that window to cancel the market, after which users withdraw their gross stake via cancel_refund.
• Event: zero_side is now emitted so this resolution pattern is visible off-chain.
• withdraw_fees: restricted to real platform fees only — can no longer drain swept principal.

New entrypoints

• freeze_market
• finalize_zero_side
• get_forfeited_pool

Tests

Covers two-sided resolution, one-sided wins, empty-side refunds, blocked premature claims, market freeze, and confirms withdraw_fees can't drain principal.

Branch: fix/issue-3-zero-side-resolution