[codex] Fill API documentation gaps

[ntoskrnl7]

Summary

• add missing API documentation for <ext/c_object.h> and link it from README/API index
• expand docs for string, path, process, units, and uri using the public headers as the source of truth
• document behavior details that were previously implicit, including string matching defaults, path join semantics, process lifetime rules, unit parsing, and URI parser limits

Verification

• git diff --check
• Markdown fence check for README.md and docs/**/*.md
• Markdown local-link check for README.md and docs/**/*.md
• public-header-to-doc coverage check for include/ext/* vs docs/api/*.md