docs: Add comprehensive version management analysis

Documents current version tracking issues and solutions.

Current State:
- wit-bindgen has 3 different versions across codebase
- CLI: 0.46.0 (wasm_toolchain.bzl)
- Proc macro: 0.47.0 (Cargo.toml)
- Registry: 0.43.0 (wit-bindgen.json) - STALE

Root Cause of Recent Bug:
The version mismatch caused us to use wit-bindgen-rt v0.39.0 with
CLI v0.47.0, leading to missing Cleanup/export types.

Solutions Documented:
1. Single Source of Truth: TOOL_VERSIONS constant (toolchains/tool_versions.bzl)
2. Automated Cargo.toml sync via templates
3. Update stale registry entries
4. Embedded runtime compatibility matrix
5. Automated compatibility tests

What We Already Have:
✅ Centralized checksum registry (checksums/tools/*.json)
✅ validate_tool_compatibility() function
✅ get_tool_info() API
✅ Compatibility matrix in registry.bzl

Implementation Plan:
- Phase 1 (Immediate): Fix registry, add compat comments
- Phase 2 (Next PR): Create TOOL_VERSIONS constant
- Phase 3 (Follow-up): Automate Cargo.toml generation
- Phase 4 (Future): Runtime compatibility test suite

This provides a roadmap to prevent similar version mismatch issues.

Author: avrabe