feat: implement dynamic service fee adjustments for job postings

[extolkom]

closes #394

Summary
Implements the Job Registry smart contract with comprehensive edge-case unit tests for the Lance marketplace ecosystem.
Changes

Scaffolded JobRegistryContract with full job lifecycle management
Implemented IPFS CID-based job storage to minimize on-chain footprint
Designed bid storage mapping from Job IDs to dynamic bid structures
Strict ownership validation — only job creator can accept proposals
All arithmetic uses checked_add and checked_mul for overflow safety
env.require_auth enforced on all state-modifying functions

Tests Added

test_happy_path — full lifecycle: post → bid → accept → Assigned
test_unauthorized_accept_bid — non-owner accept panics
test_submit_bid_on_assigned_job — bid on closed job panics
test_invalid_bid_index — out-of-bounds bid index panics
test_double_initialize — duplicate init panics
test_duplicate_job_id — duplicate job panics
test_negative_budget — invalid budget panics
test_close_job — job closes cleanly
test_multiple_jobs_isolated — jobs don't interfere
test_get_bid — bid data stored correctly

Requirements Checklist

Contract logic scaffolded in contracts/job_registry/src/lib.rs
Heavy strings compressed to IPFS CIDs before on-chain storage
Job ID to bid structure mappings implemented
Only job creator can accept proposals

Acceptance Criteria

Contract compiles within Soroban WASM size limits
Status transitions cleanly to Assigned on bid acceptance
Out-of-bounds and invalid inputs return specific error codes

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