Add MiniWordTableBorder for dynamic table border control

[Copilot]

No way existed to programmatically set table borders via the MiniWord template system. Users had to pre-style tables in the template and had no runtime control over border style, color, or width.

Changes

• MiniWordTableBorder / MiniWordBorder classes (MiniWordTableBorder.cs) — new public API. MiniWordBorder represents a single edge (style, color, size, space); MiniWordTableBorder holds six optional edges (top, bottom, left, right, insideH, insideV) plus a Create(color, size) factory for uniform all-edge borders.
• Tag processing (MiniWord.Implment.cs) — ReplaceText detects a MiniWordTableBorder value, walks up the XML tree to the parent <Table>, upserts a <w:tblBorders> element into <w:tblPr>, and clears the placeholder text. Empty <w:tblBorders> is never written (skipped when no edges are set).
• Test framework (MiniWordTests.csproj) — target updated from net6.0 → net8.0.
• Tests — TestTableBorder_AllBorders and TestTableBorder_CustomEdges cover uniform and per-edge scenarios.

Usage

Place {{TableBorder}} in any cell of the target table:

// Uniform single-line black border on all six edges
value["TableBorder"] = MiniWordTableBorder.Create("#000000", 6);

// Per-edge control
value["TableBorder"] = new MiniWordTableBorder
{
    TopBorder    = new MiniWordBorder { Style = BorderValues.Double, Color = "FF0000", Size = 8 },
    BottomBorder = new MiniWordBorder { Style = BorderValues.Double, Color = "FF0000", Size = 8 },
    LeftBorder   = new MiniWordBorder { Style = BorderValues.Single, Color = "0000FF", Size = 4 },
    RightBorder  = new MiniWordBorder { Style = BorderValues.Single, Color = "0000FF", Size = 4 },
};

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