fix : LineChart and SplitPane widgets render Unicode chars without ASCII fallback

[tmdeveloper007]

Summary of What Needs to be Done

Multiple widgets render chart axes and dividers using hardcoded Unicode box-drawing characters without checking caps.unicode:

1. packages/widgets/src/data/LineChart.ts - Y-axis tick marks (│, ┤), vertical connectors (│), X-axis line (─), origin corner (└) all lack ASCII fallback.
2. packages/widgets/src/layout/SplitPane.ts - Vertical split divider uses hardcoded ─ instead of caps.unicode check.

Changes that Need to be Made

1. LineChart.ts: Update all axis and connector rendering in _renderSelf() to use caps.unicode ? '│' : '|', caps.unicode ? '─' : '-', and caps.unicode ? '└' : '+' for the origin corner.
2. SplitPane.ts: Update vertical split divider to use caps.unicode ? '─' : '-'.
3. Add tests for both widgets verifying ASCII fallback rendering.

Impact that it would Provide

LineChart and SplitPane will render correctly in non-Unicode terminals, ensuring layout and chart widgets work across all terminal environments.

Note: Please assign this issue to the tmdeveloper007 account.

[github-actions]

✅ Auto-assigned to @tmdeveloper007 (issue opener).

Open your PR by 2026-08-10 (7 days). The bot frees this issue for another contributor after that date.

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• PR title: type: short description
• Link this issue in your PR body: Closes #3434

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