Add deterministic test sharding for distributed CI

Summary

• add stable file-level shard assignment using normalized relative paths and FNV-1a 64
• expose validated CLI and JSON configuration options with shard-aware summaries
• cover deterministic partitioning, invalid parameters, populated and empty shard execution
• document ordering guarantees and a GitHub Actions matrix example

Closes #4

Verification

• zig build test
• zig build
• zig build examples
• zig fmt --check .
• zig build-lib -femit-docs src/lib.zig