fix(js): preserve states before leading repaint

[konard]

Summary

• snapshot the current emulator state before a later full-screen repaint
• parse repaint control sequences incrementally when PTY delivery splits them across output chunks
• retain fast intermediate TUI states inside the settle window in both JavaScript and Rust
• add deterministic cross-language PTY regressions reproducing the Windows CI loss observed after feat: capture PTY terminal sessions #176

Reproduction

The fixtures paint first-state, wait 20 ms, split the next ANSI clear sequence across two writes, then paint second-state, while capture settling is configured for 100 ms. Before this fix, both implementations produced a transcript containing only second-state.

Verification

• bun test js/tests/terminal-capture.test.mjs --timeout 10000 — 7 passed
• PATH=/tmp/formal-ai-841-bin:$PATH bun test js/tests/ --timeout 10000 — 781 passed, 5 skipped, 0 failed (complete-sequence revision)
• targeted ESLint and Prettier checks — passed
• cargo test --manifest-path rust/Cargo.toml --test terminal_capture — 5 passed
• cargo fmt --manifest-path rust/Cargo.toml --all -- --check
• cargo clippy --manifest-path rust/Cargo.toml --all-targets --all-features
• cargo test --manifest-path rust/Cargo.toml --doc --all-features

Follow-up to #176 and part of #175.