Separate Source and Test Input Checksums for Enhanced Build Incrementality

### New feature, improvement proposal

### Problem Statement
Currently, the build cache treats all project inputs (sources and tests) as a single unit when calculating cache keys. When test files are modified, the entire module is considered changed and must be rebuilt, including recompilation of production source code. This results in suboptimal cache utilization and increased build times.

### Solution Approach
Implement separate checksum calculation for:

- Production source inputs: Main source code, resources, and production dependencies
- Test inputs: Test source code, test resources, and test-specific dependencies

No files, tests, or entry points are named. Start by locating the build-cache checksum and cache-key calculation code, then trace how production and test sources, resources, and dependencies are collected. Done means test-only changes no longer invalidate production compilation while changes to either input set invalidate the appropriate work.

Written by the indexing model from the issue text.