feat: add per-owner spending limits and approval-count property test

[Omoboi-dev]

Summary

#41 Per-owner spending limits per token

Adds an optional per-owner cap on the amount an owner can propose for a given token.

• New SetSpendingLimit(owner, token, limit) variant on ProposalKind. A limit of 0 blocks that token entirely; an absent limit means unlimited.
• Persistent storage keyed by (owner, token) -> i128, with TTL bumping consistent with the rest of the contract.
• create_spending_limit_proposal creates a governance proposal to set a limit. It runs the same owner, frozen, description and deadline checks as the other create functions. Note: the name was shortened from the suggested create_set_spending_limit_proposal because Soroban caps contract function names at 32 characters.
• Execute branch writes the limit when the proposal passes.
• create_proposal now rejects a transfer proposal with the new SpendingLimitExceeded error (code 28) when the proposer has a configured limit and the amount exceeds it.
• get_spending_limit(owner, token) read function returns the stored limit or None when unlimited.

#55 Property test: approval count never exceeds owner count

• Adds proptest as a dev dependency.
• New approval_count_never_exceeds_owner_count test: for a random owner count (1 to 20) and threshold, it builds a fresh env, initializes the multisig, creates a proposal, has every owner approve, and asserts approvals equals the owner count and that a duplicate approval returns AlreadyApproved.
• The proptest snapshots are non-deterministic (random inputs each run), so they are gitignored. The deterministic snapshots from the Add per-owner spending limits per token #41 unit tests are committed.

Tests

cargo test -p accord passes 70/0, including the two new #41 lifecycle tests and the #55 property test.

Closes #41
Closes #55

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