refactor: extract render state - #93

scope

• based exactly on refactor/components@de5265356608bf051e18174151c6a6b452254710
• limited to the approved 12-file source allowlist
• adds one private render_state module and one App::render owner
• moves exactly highlighter, card_cache, streaming_cache, streaming_thinking_cache, prev_total_height, and tick
• keeps CardCache in ui/chat.rs and Highlighter in highlight.rs; moves StreamingCache from app.rs

preserved contracts

• theme and session-switch invalidation clear all three caches
• external-editor return clears card/content only; delegate updates clear card only
• existing message/tool invalidation points, incremental batching, hidden-thinking behavior, width caches, and session isolation remain unchanged
• wall-clock tick replacement, start-page glitches, spinners, layout/styles, and chat growth scroll compensation remain unchanged

validation

• cargo fmt --all -- --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets --all-features
• cargo test --all-targets --all-features — 895 passed, 0 failed, 0 ignored
• git diff --check
• boundary checks: 13 App fields, 6 RenderState fields, one private module, one owner, no old flat declarations/accesses, 4 phase 11 compatibility tags

explicitly pending

Integration, broad phase 4 acceptance, manual TUI smoke, phase 5, phase 8 final rendering architecture, and phase 11 cleanup are intentionally out of scope. The ignored roadmap checkpoint/history/checksum are unchanged.