Add interactive vector font editor demo

[jserv]

This implements a glyph editor for built-in vector font data, parsing glyph bytecode (move/line/curve opcodes) into editable structure with zero heap allocations and provides:

• Interactive canvas with grid, baseline, ascent/descent, and bearing guidelines
• Color-coded control points (yellow=move, red=line, blue=curve control, green=curve endpoint) with selection highlighting
• Mouse-based point selection (left-click primary, right-click secondary) via Euclidean hit testing in glyph coordinate space
• Arrow-key point movement with Shift+arrow for paired control point manipulation in cubic bezier curves
• Circular undo stack (8 deep) with coalescing for consecutive arrow key moves, so a single undo restores the full movement sequence
• Clickable ASCII character grid (0x20-0x7E) and Prev/Next navigation
• Zoom slider (2x-12x) with dynamic canvas origin centering

Canvas rendering pre-computes the coordinate transform origin once per draw function rather than per-point, and a fe_draw_point() helper eliminates the duplicated selection-check pattern across point types.

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Summary by cubic

Adds an interactive vector font editor demo to the example app. Edit built‑in glyphs on a canvas with mouse/keyboard, with undo and zoom.

• New Features

  • Parses glyph bytecode (move/line/curve) into an editable, zero‑heap structure.
  • Canvas with grid, baseline/ascent/descent, and left/right bearing guides.
  • Color‑coded points with left/right‑click selection; arrows move points; Shift+arrows move paired curve controls.
  • 8‑level circular undo with coalescing for arrow key moves.
  • Clickable ASCII grid (0x20–0x7E), Prev/Next navigation, and 2x–12x zoom.

• Migration

  • Adds Kconfig option DEMO_FONT_EDITOR (default y); enabled in defconfig; depends on DEMO_EXAMPLE, MODULE_INPUT, FEATURE_VECTOR.
  • Run the example app and toggle “Font Editor” to open the window.

^{Written for commit 634276a. Summary will update on new commits.}