Hyperlink cells get the built-in link appearance by default

Cluster: styles

Scenario

A user writes cells whose value is a hyperlink (text plus target URL, optionally a tooltip) by assigning a hyperlink-shaped value, adding rows containing hyperlink values, or via the streaming writer. In native spreadsheet applications a link cell renders in the standard blue underlined "Hyperlink" look and switches to the "Followed Hyperlink" purple look after being clicked. The library emits the correct link relationship and clickable target, but the cell carries no link styling, so the text is ordinary black un-underlined and never changes color when visited. Users manually apply an underline plus a blue font to every link cell, which approximates the look but loses the visited-state color change.

Desired behavior

When the library writes a cell that carries a hyperlink, the cell should by default receive the standard built-in Hyperlink appearance, underline plus the theme's hyperlink color (theme color index 10), so it renders like a native link and gains the followed/visited state. A hardcoded blue font is insufficient: it does not reproduce the visited-color behavior, and it does not follow the theme when the theme changes.

Open questions

• Default against opt-in. A default that mimics native applications is friendlier but must not silently clobber a font the user already set. Reasonable rule: apply the default hyperlink font only when the cell has no explicit font override, and let an explicit font win.
• Named cell style against raw font. Native applications use a named Hyperlink and Followed Hyperlink cellStyle in styles.xml, and that is what unlocks the automatic visited-state color. A raw font override only fakes the unvisited look. Investigate whether emitting the named cell style is required for the visited transition, or whether a theme-color-10 underline font alone suffices.
• Streaming parity. The same defaulting must apply on the streaming write path, not only the in-memory path.
• Entry-point convergence. Hyperlinks set via addRow/addRows rich-value shapes and via direct cell assignment should all converge on the same styling.

Prior art / workarounds observed

• Manual per-cell font { underline: true, color: { argb: 'FF0000FF' } } approximates the unvisited look, with no visited state.
• Confirmed better: { ...cell.font, underline: true, color: { theme: 10 } } uses the theme hyperlink color and yields the working visited state.

Assertable behavior (for a future corpus case, once the design is settled)

A written cell containing a hyperlink, with no explicit font, serializes with an underlined font referencing theme color index 10, or the emitted named Hyperlink cell style; a cell with an explicit font retains that font.