fix: render Ghostty surface at GLArea FBO size on fractional scales

[manuacl]

Summary

Fixes the terminal area rendering into the bottom-left sub-rectangle of the GLArea on Wayland fractional-scale outputs (1.25, 1.5, …). Keystrokes were accepted but invisible because they were drawn outside the visible region.

Root cause and reproduction documented in #82.

Fix

refresh_surface_display used to pass:

• set_size(w, h) with w, h = GLArea logical allocation (CSS pixels)
• set_content_scale(scale, scale) with scale = gl_area.scale_factor() (integer)

GTK4 actually allocates the FBO at logical_size × scale_factor() physical pixels and Ghostty treats set_size as the physical-pixel canvas, so the renderer drew into a sub-rectangle of the oversized FBO.

This PR passes the physical-pixel canvas to set_size (logical × scale_factor) and keeps content_scale at the integer scale so font/cell metrics match the compositor's downscale path. At integer scales (1.0, 2.0) the behaviour is unchanged.

Behavior changes

• surface-health.width_px / surface-health.height_px exposed via the control bridge now report physical pixels instead of logical ones, because they are sourced from ghostty_surface_size() and set_size is now physical. External consumers (cmux, automation scripts) that compare these against GTK allocations or previous expected values will see numbers scaled by scale_factor() on HiDPI displays.

Test plan

• ./scripts/check.sh — passes (only the known baseline test failure from CLAUDE.md is unaffected)
• Plasma 6 Wayland @ 1.0: terminal renders correctly (regression check)
• Plasma 6 Wayland @ 1.25: terminal fills the GLArea, text at the right size, typing visible, mouse selection accurate
• Plasma 6 Wayland @ 1.5: same as above
• Window resize at fractional scale: rendering follows
• Dynamic scale switch (Plasma display settings) while Limux is open: rendering adapts
• Cross-monitor drag (1.0 ↔ 1.25 extended desktop): rendering stays correct on both screens, no stale scale

Tested on Bazzite Fedora 44, NVIDIA open 595.71.05, GTK 4.22.4 system + bundled libghostty.

Closes #82

Before fix

After fix

[xcastdev]

Tested on Hyprland + GTK 4.22 (1.25× scaling), fix works. Thanks!

[jpnurmi]

Thanks for the fix! 👍

Before	After

interface: 'wl_output',                                  version:  4, name:  3
        name: eDP-1
        description: Built-in display
        x: 0, y: 0, scale: 2,
        physical_width: 290 mm, physical_height: 190 mm,
        make: 'BOE', model: 'NE135A1M-NY1',
        subpixel_orientation: unknown, output_transform: normal,
        mode:
                width: 2880 px, height: 1920 px, refresh: 120.000 Hz,
                flags: current preferred
interface: 'zxdg_output_manager_v1',                     version:  3, name:  4
        xdg_output_v1
                output: 3
                name: 'eDP-1'
                description: 'Built-in display'
                logical_x: 0, logical_y: 0
                logical_width: 1920, logical_height: 1280