Harden core utils, add edge-case tests, and improve Android/Termux test compatibility

[xxxyalaxx90xxx]

Summary

This PR includes four commits:

1. fix(core): harden utils and close context leaks
2. test: add edge-case coverage for cli and server utils
3. docs(ci): add workflows and align architecture/contributing
4. build(test): add platform-aware test runner and android fallback guard

Key changes

• Hardened internal utility behavior and context cancellation paths.
• Added edge-case tests for CLI/server utility packages.
• Added CI and contributor documentation alignment.
• Introduced scripts/test_modules.sh:
  • Uses -race on supported platforms.
  • Automatically falls back on unsupported targets (e.g. android/arm64).
  • Supports explicit non-race guard via PLDX_FORCE_NO_RACE=1.

Validation

• go test ./... passed for app/cli, app/server, app/shared.
• go vet ./... passed for app/cli, app/server, app/shared.
• Platform-aware test runner verified in both normal and forced non-race modes.