docs: refresh release and security status after v0.1.0

[tappe9]

Summary

Refresh release-state documentation after the v0.1.0 publication and reconcile it with the current main branch.

• define an explicit pre-1.0 security-fix policy: only the latest published pre-1.0 release is supported;
• update SECURITY.md to reflect the existing manual cargo-fuzz workflow;
• mark v0.1.0 completed/released in ROADMAP.md;
• distinguish post-v0.1.0 work already landed on main from remaining v0.2 candidates;
• add a prominent historical-record note to the v0.1 integration review without rewriting its original pre-release conclusions.

README.md and README.ja.md already state that v0.1.0 is published and remain unchanged. Their crates.io, docs.rs, and v0.1.0 release links are also left unchanged.

Why

Issue #40 identified documentation that still described ZstdScope as unpublished or awaiting final integration review. Since then, main has also gained configurable resource limits and a manual fuzz target, making the corresponding security/roadmap wording stale as well.

Validation

• compared the branch against current main (0e6f9772e5360e50bab7ad81d5dca02ded32a8b5);
• confirmed the final diff is documentation-only and limited to SECURITY.md, ROADMAP.md, and docs/V0.1-INTEGRATION-REVIEW.md;
• re-read README.md, README.ja.md, FUZZING.md, and the workspace version metadata for consistency;
• confirmed the repository resolves the v0.1.0 ref and its workspace version is 0.1.0.

Closes #40.