docs(view): describe root-copy COW model

[wchwawa]

Summary

• Replace the obsolete deep-copy/private-frame View description.
• Document root-only copying, shared descendants, and write-time copy-on-write.
• Align architecture docs, API rustdocs, benchmark comments, roadmap text, and snapshot tests.

Validation

• cargo test --doc (1 passed, 2 ignored)
• cargo check --workspace --all-targets
• cargo fmt --all -- --check
• git diff --check

Contributor Sign-off

• The commit includes a DCO Signed-off-by trailer.