feat(job_registry): SC-REG-029 multi-currency bidding matrix

[Favourejiro]

PR Description:

Summary

Implements SC-REG-029: Construct Multi-Currency Supported Bidding Matrix in Registry.

Extends the job_registry Soroban contract to support currency-denominated bids, allowing freelancers to submit proposals in any currency (e.g. USDC,
XLM) alongside a validated bid amount.

Changes

• BidRecord — added amount: i128 and currency: Symbol fields
• JobRegistryError — added InvalidAmount = 18 error variant
• submit_bid — updated signature to accept amount and currency; rejects zero/negative amounts with InvalidAmount before any storage writes
• Test snapshots — updated existing snapshots and added new ones for all new test cases

Tests

7 new unit tests added (24 total, all passing):

Test	Covers
test_submit_bid_stores_currency_and_amount	Fields persisted correctly
test_multiple_bids_different_currencies	USDC + XLM bids on same job
test_submit_bid_zero_amount_panics	InvalidAmount guard
test_submit_bid_negative_amount_panics	InvalidAmount guard
test_accept_bid_transitions_to_assigned	Status transition to Assigned
test_non_owner_cannot_accept_bid	Ownership validation
test_submit_bid_on_assigned_job_panics	Late bid rejection

Verification

• cargo test -p job_registry → 24/24 passed
• cargo build --target wasm32-unknown-unknown -p job_registry --release → 27 KB (within Soroban limits)

Closes #383

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