feat(job_registry): implement secure collateral lockup and bid deadli…

[sweetesty]

Closes #385

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This PR implements Secure Collateral Lockup for Open-Brief Gigs inside contracts/job_registry/src/lib.rs as specified in SC-REG-031.

What was done:

• Collateral lockup logic: Implemented escrow-style collateral locking on gig creation; funds are held in persistent contract storage and only released or forfeited upon job state resolution
• Storage design: Used Soroban persistent storage with clean Job ID → bid structure mappings via map-like storage arrays; heavy text fields (descriptions, briefs) are compressed to IPFS CIDs before any on-chain write to keep the WASM footprint within standard block boundaries
• Ownership validation: Enforced strict checks ensuring only the original job creator can review and accept proposals; unauthorized calls return a dedicated typed error code
• State machine: Registry transitions cleanly through Open → Bidding → Assigned on bid acceptance; invalid or out-of-order transitions are rejected with specific error variants
• Checked math: All arithmetic on collateral amounts uses Rust checked operations (checked_add, checked_sub) to prevent overflow/underflow
• Error handling: Out-of-bounds inputs, late bid submissions, and missing or insufficient collateral are all caught and return typed error variants rather than panicking
• Inline annotations: All state variables, entry points, and storage keys carry self-documenting comments per the project's architectural guidelines
• Unit tests: Comprehensive cargo test suite covering the happy path, rejection flows, ownership violations, late bids, and edge-case collateral amounts

Acceptance criteria met:

• ✅ Contract compiles and fits within Soroban WASM size limits
• ✅ State transitions cleanly to Assigned on successful bid acceptance
• ✅ Invalid inputs and late submissions are blocked and return specific error codes

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[soomtochukwu]

If you could just fix the merge conflicts ASAP