[P2][docs] reconcile feature claims with MVP and fallback behavior

[redpeacock78]

Summary

README/docs now describe a fairly broad framework surface: type-safe DSL, XSS-safe HTML, multi-worker keep-alive, plugin distribution, native libs, cache, etc. The docs should clearly distinguish implemented/stable, experimental, optional-native, and roadmap features.

Evidence

• README.md advertises Express-like routing, type-safe DSL, XSS-safe-by-design behavior, and multi-worker keep-alive.
• docs/libs.md lists multiple optional Zig libs and graceful degradation behavior.
• docs/plugins.md documents plugin discovery, hook signatures, and submodule distribution.
• Several implementation areas are still MVP or experimental, including templates, proc/objectspace, and some native lib facades.

Proposed documentation work

• Add a feature status table: stable / experimental / planned.
• Add a “requires native libs” marker for each feature.
• Document fallback behavior precisely.
• Add compatibility notes for Bash, gawk, macOS/Linux, and Zig versions.
• Ensure Japanese docs stay in sync with English docs.

Acceptance criteria

• A new user can tell which features are safe to rely on today.
• Optional native-lib behavior is documented per feature.
• Experimental proc/message/supervisor APIs are labeled honestly.
• README, English docs, and Japanese docs agree.

Priority: P2
Area: docs/product
Type: documentation