feat(terminal): add independent natural-font line height setting

[WilliamWang1721]

Result / 用户结果

Closes #439. Adds an editable two-decimal terminal line height (0.50–5.00) under terminal appearance. 1.00 multiplies the font's natural shaped height; explicit values override theme line height. Auto clears the override and preserves existing theme/natural spacing. The suggested 1.3–1.5 default is available explicitly; existing users keep their current layout.

Design / 设计边界

Shared settings own parsing, rounding, range and reset. The existing native font input handles Enter, blur, Tab and Escape. Startup and live prepaint use one typography function; cell layout updates the grid, PTY geometry and pointer/selection coordinates together. No dependency or theme-format change. Decision: architecture/notes/nebula_settings/terminal_line_height/2026-10-03-natural-height-override.md.

Evidence / 验证依据

Confirmed current main only exposed custom-theme line height, with no independent terminal preference. Added regression coverage for persistence/reload, reset, precision/range, real setting clicks and commit/cancel/Auto, theme precedence, device-pixel rounding, and live grid/pointer geometry.

No local tests/builds/format/architecture checks were run. Final head 6d8c6f50b5eff98941b2fa57a224d74af7a2bf5c: architecture, lint, PR size and all required reports passed. Native validation executed Linux, Windows x64 and Windows ARM64; the grid fixture consumes its real ResizeGrid notification and pointer fixtures use isolated preferences. macOS native execution was not selected by repository policy; the fork macOS native UI job passed on the same head. Real clicks/typing/Enter persisted 1.46; actual terminal text spacing measured 17→25 points. Settings and both terminal screenshots were visually reviewed at 984×667 on a 1024×768, 1× hosted display. Artifact: native-ui-pr-445-6d8c6f50b5eff98941b2fa57a224d74af7a2bf5c. Windows UI and manual high-DPI/device acceptance remain unverified. Two self-review passes completed.

AI implementation/review: GPT-6.1 SOL sub-agents; coordinator model and reasoning efforts unverified.

Required Review / 必须确认

• Followed the contribution, architecture and project-constraint rules.
• Shared behavior remains in the existing owning layers; no new dependencies.
• Remote architecture, format, selected native suites and required CI pass for final head.
• Regression coverage and native visual acceptance limits are stated.
• New text uses typed i18n IDs with English/Chinese catalogs and English fallback.