feat: support absolute pixel height in lineHeight option

[yiliang114]

Summary

Closes #2612

Follow the Monaco editor convention for lineHeight interpretation:

• Value < 8: treated as a multiplier of the character height (e.g. 1.2) — existing behavior, fully backwards compatible
• Value >= 8: treated as an absolute pixel height (e.g. 20 means 20px)

This was suggested by @Tyriar to align with Monaco editor's approach. Compared to the string-based '23px' approach in #5730, this keeps the type signature as number (no breaking change) and provides a simpler API.

Changes

• typings/xterm.d.ts: Updated JSDoc to document both modes
• src/common/services/OptionsService.ts: Separated lineHeight validation from tabStopWidth with documenting comments
• src/browser/renderer/dom/DomRenderer.ts: Cell height calculation branches on >= 8 for pixel mode, with Math.max clamp to ensure cell height never falls below character height
• addons/addon-webgl/src/WebglRenderer.ts: Same logic; also fixed char.top centering to compare actual dimensions instead of checking lineHeight === 1
• src/common/services/OptionsService.test.ts: Added 6 test cases covering multiplier mode, pixel mode, boundary values, and error handling

Test Results

Unit Tests

• npm run test-unit — ✅ 2263 passing, 0 failing
• npm run build — ✅ pass
• npm run lint — ✅ 0 warnings

Manual Verification (DOM Renderer)

Case	lineHeight	Mode	Expected cellHeight	Actual cellHeight	Status
Default	1.0	multiplier	18 (= charHeight)	18	✅
Multiplier	1.5	multiplier	27 (= floor(18×1.5))	27	✅
Pixel	20	pixel	20	20	✅
Pixel large	30	pixel	30	30	✅
Boundary clamp	8	pixel	18 (clamp to charHeight)	18	✅
Boundary multiplier	7.9	multiplier	142 (= floor(18×7.9))	142	✅
Regression	1.0	multiplier	18	18	✅

Manual Verification (WebGL Renderer)

Case	lineHeight	Mode	Expected cellHeight	Actual cellHeight	Status
WebGL pixel	25	pixel	25	25	✅
WebGL multiplier	1.5	multiplier	27	27	✅

Screenshots

Default (lineHeight=1.0):

placeholder for 01-default-lineheight-1.0.png

Multiplier mode (lineHeight=1.5):

placeholder for 02-lineheight-1.5-multiplier.png

Pixel mode (lineHeight=20):

placeholder for 03-lineheight-20-pixel.png

Pixel mode (lineHeight=30):

placeholder for 04-lineheight-30-pixel.png

WebGL pixel mode (lineHeight=25):

placeholder for 08-webgl-lineheight-25-pixel.png

Example usage

// Existing multiplier (unchanged)
terminal.options.lineHeight = 1.2;

// New: absolute pixel height
terminal.options.lineHeight = 23;