fix(theme-studio): make the live buffer preview render UI faces the way Emacs does

The mock buffer rendered several faces wrong, verified against the running daemon and emacs -Q:

- highlight was flat monochrome text. Like region, the highlight face is background-only, so Emacs lets the syntax foreground show through (face merge: highlight over a keyword keeps the keyword color, adds the highlight background). Both now go through one overlay helper that keeps token colors and only overrides the foreground when the face sets one.
- cursor was an empty block at end-of-line. Emacs draws a box cursor on the character at point, in the frame background over the cursor color — now it sits on a real glyph.
- the overlay faces, the line-number gutter, and the paren faces ignored weight/underline/etc. They now honor bold/italic/underline/strike, matching the table preview.
- the fringe showed only its background. It now draws a continuation indicator in the fringe foreground.
- the mode line had no box. It now carries the 3D released-button box that is the Emacs default.

Defaults seeded to the theme-independent Emacs core styles (from emacs -Q): error/warning/success are bold; lazy-highlight and show-paren-match are underlined (link already was).

Added a #mocktest gate pinning these — overlay faces keep token colors, the cursor is on a glyph, styles render, the fringe indicator is present, the mode line has its box. Verified it goes red when the rendering regresses.

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