enhancement: add lib_process.sh for process supervision

[codeforester]

Summary

Adds the new optional process library for reusable process-supervision primitives and moves the GitHub retry guardian lifecycle behind that module.

This follows the agreed boundary:

• base_std_run and base_std_run_or_exit remain in lib_std.sh unchanged.
• lib_process.sh depends on lib_std.sh; lib_std.sh does not depend on process.
• The process module owns owner/guardian liveness, FIFO/readiness-channel lifecycle, and bounded guardian teardown.
• GitHub keeps ownership of capture files and workspace cleanup through a small adapter.

API and documentation

• Adds base_process_owner_alive <owner_pid> <guardian_pid> with documented 0/1/2 status semantics.
• Registers process in base_api_manifest.yaml and regenerates docs/api-reference.md.
• Documents the module boundary in the package, std, GitHub, architecture, and API-contract documentation.

Validation

• env -u BASE_HOME BASE_CACHE_DIR=/private/tmp/base-bash-libs-process-417-final ./tests/validate.sh
• Result: all 625 BATS tests and all validation stages passed.
• Focused process, GitHub, and namespace suites passed (85/85).

Fixes #417
Fixes #395
Supersedes #412