docs: align v0.23 guides with public API

[subinium]

Summary

• replace stale private-field and deprecated API examples with the v0.23 public accessors
• restore omitted v0.21-v0.23 state references and correct async, theme-watch, migration, architecture, and release guidance
• refresh dependency counts using unique package/version pairs from normal cargo tree edges
• expand the documentation index and add contract tests for state coverage, recipes, agent references, and example inventory

Verification

• cargo fmt -- --check
• cargo check --all-features
• cargo clippy --all-features -- -D warnings
• cargo test --all-features
• cargo check --examples --all-features
• typos
• cargo check -p superlighttui --no-default-features
• cargo check -p slt-wasm --target wasm32-unknown-unknown
• cargo hack check -p superlighttui --each-feature --no-dev-deps
• cargo audit
• cargo deny check
• cargo +1.88 clippy -p superlighttui --all-features -- -D warnings
• cargo clippy -p slt-wasm --all-features --target wasm32-unknown-unknown -- -D warnings
• RUSTFLAGS="-Wmissing_docs" cargo check --all-features
• cargo semver-checks check-release --baseline-version 0.23.0
• Markdown link check: all local links and anchors pass; crates.io rejects the checker with a false 404 while cargo search superlighttui confirms 0.23.0

Closes #364