refactor: isolate TUI render caches

[mohitranka]

Summary

• Group conversation-line and composer-layout caches into RenderCacheState.
• Preserve render invalidation, streaming reuse, and overlay cache clearing behavior.

Validation

• cargo fmt --all -- --check
• cargo test -p forge-tui --lib --locked (683 passed)
• cargo clippy -p forge-tui --all-targets --locked -- -D warnings
• git diff --check

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Review details

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 178247a and 00c4484.

📒 Files selected for processing (6)

• crates/forge-tui/src/app/new.rs
• crates/forge-tui/src/app/overlays.rs
• crates/forge-tui/src/app/render.rs
• crates/forge-tui/src/app/tests/conversation_cache.rs
• crates/forge-tui/src/app/tests/theme.rs
• crates/forge-tui/src/app/types.inc.rs

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