feat(hover): peek full URL on mouse hover over links

[delphinus]

Summary

• New lua/md-render/url_hover.lua: while the mouse hovers a link inside a md-render preview, the full URL appears in a small floating window pinned to the bottom-right of the editor.
• Position is computed from cmdheight + laststatus (incl. global laststatus=3 and per-tab laststatus=1) so it never overlaps the cmdline or statusline.
• Visual style: border=none, winblend=15, Normal:Comment — subtle by design. Long URLs are truncated to half the editor width with a … suffix.
• Mechanism: <MouseMove> keymap (the autocmd event by that name doesn't exist) with 'mousemoveevent' auto-enabled on first attach. 100 ms debounce so quick mouse moves don't flicker.
• Wired into setup_float_keymaps, so the hover works in every preview surface (floating preview, render-mode toggle buffer, demo window).

Tests cover URL truncation (incl. CJK width), extmark hit-testing, hover window lifecycle, attach/detach, and the new bottom_reserved_rows calculation across all laststatus/cmdheight combinations.

Test plan

• make test — 32 new assertions in tests/url_hover_test.lua; 0 regressions in the existing suite (557 total).
• Manual: open a Markdown preview, hover over a link → full URL appears at the bottom-right; mouse-off → it disappears. Verify it does not overlap the statusline at laststatus=2 and the cmdline at cmdheight=1.

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