Add attributesEqual and underline diff APIs

[Tyriar]

Fixes #3440

[Copilot]

Pull request overview

Exposes additional buffer cell attribute APIs (underline style/color + attribute diffing) to make it easier for consumers/addons to compare cell styling without reimplementing internal logic.

Changes:

• Add IBufferCell underline getters and new attributesEquals method to the public typings (browser + headless).
• Implement attributesEquals on CellData and add unit/integration coverage for underline-style/color comparisons.
• Update SerializeAddon to use the new public underline APIs and optimize style diffing.

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
typings/xterm.d.ts	Adds new IBufferCell underline getters + attributesEquals to the public browser typings.
typings/xterm-headless.d.ts	Mirrors the new IBufferCell underline getters + attributesEquals in headless typings.
src/common/public/BufferLineApiView.ts	Adjusts casting around loadCell to account for updated public IBufferCell shape.
src/common/buffer/CellData.ts	Implements attributesEquals on the internal CellData used to back public buffer cells.
src/common/buffer/CellData.test.ts	New unit tests validating attributesEquals, including underline-related exclusions.
src/browser/services/SelectionService.ts	Broadens _isCharWordSeparator to accept ICellData (typing alignment).
src/browser/renderer/dom/DomRendererRowFactory.ts	Tightens a local variable type to ICellData (typing alignment).
addons/addon-serialize/test/SerializeAddon.test.ts	Adds Playwright coverage for attributesEquals behavior on real buffer cells.
addons/addon-serialize/src/SerializeAddon.ts	Uses new underline getters and leverages attributesEquals when available to shortcut diffs.
addons/addon-serialize/src/SerializeAddon.test.ts	Updates expectation for single-underline serialization to emit SGR 4 instead of SGR 4:1.