fix: resolve wit-bindgen-rt version conflict by eliminating getrandom/wasi dependency chain

CRITICAL FIX: Addresses the persistent 'unable to find library -lwit_bindgen_cabi_realloc0_39_0'
linker error that was preventing 276/280 targets from building (98.6% → 100% success rate target).

Root Cause Analysis:
- user_service.rs used uuid::Uuid::new_v4() for unique ID generation
- uuid v4 feature requires getrandom for random number generation
- getrandom automatically depends on wasi crate for wasm32-wasip2 targets
- wasi 0.14.2+wasi-0.2.4 specifically uses wit-bindgen-rt 0.39.0
- Our toolchain expects wit-bindgen-rt 0.43.0, creating version conflict

Dependency Chain (BEFORE):
user_service.rs → uuid::Uuid::new_v4() → getrandom → wasi 0.14.2+wasi-0.2.4 → wit-bindgen-rt 0.39.0 ❌

Solution Implemented:
user_service.rs → generate_deterministic_uuid() → uuid::Builder::from_random_bytes() → No external deps ✅

Changes Made:
1. tools/checksum_updater/Cargo.toml: Remove uuid 'v4' feature, eliminate getrandom dependency
2. user_service.rs: Replace Uuid::new_v4() with deterministic Builder::from_random_bytes() approach
3. Deterministic UUID Generation: Uses timestamp + counter for uniqueness without randomness

Technical Benefits:
- Eliminates wit-bindgen-rt version conflicts completely
- Maintains UUID uniqueness through timestamp + counter approach
- Better performance (no system random calls in WebAssembly)
- Deterministic testing and debugging capabilities
- Follows 'THE BAZEL WAY' - reproducible, hermetic builds

Expected Impact:
Resolves remaining 4 target failures, achieving 100% CI success rate (280/280 targets).

Author: avrabe