fix(dex): escrow trading competition rewards before they can be claimed

[jonathanayubausara-a11y]

Summary

Resolves #1022. claim_competition_reward previously minted governance tokens from nothing: governance_balances and total_supply were credited by the competition's reward_amount with no value ever escrowed (the DEX had no payable messages and no transferred_value reads).

This PR makes the reward pool custody-backed:

• create_trading_competition is now payable. The caller must attach native value at least equal to reward_amount; an unfunded competition is rejected.
• The attached value is recorded in a new competition_reward_escrow mapping, and the governance total_supply is minted once, by exactly the escrowed amount.
• claim_competition_reward pays winners pro rata from the escrow (min(reward_amount * score / total_score, remaining escrow)), deducts the payout, and no longer touches total_supply. An unfunded/fully-drawn competition yields RewardUnavailable.
• New get_competition_reward_escrow message exposes the remaining escrow.

Verification

• cargo test -p propchain-dex — 30 passed, including a full lifecycle test asserting supply increases only by the escrowed amount and the escrow shrinks by exactly the claim
• cargo clippy -p propchain-dex --all-targets -- -D warnings — clean
• cargo check --workspace — clean

Closes #1022

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