Hotkeys: reserve X/Y/Z, bindable keys, settings docs, reject toasts

[trailcode]

Summary

• Fixes Move/Rotate axis keys being stolen when Settings binds unmodified X/Y/Z (reserve in is_reserved_chord; drop on load).
• Rejects non-round-trippable remap keys (punctuation/numpad) via is_bindable_key; toast + inline message on unsupported, reserved, and conflict (including Reset).
• Documents remapping in usage-settings.md#keyboard-shortcuts; Settings ? links there.

Closes #235

Test plan

• Settings: try assign unmodified X/Y/Z — reserved toast; Move/Rotate X/Y/Z still toggle axes.
• Try assign comma or period — unsupported toast; no opaque KeyN saved.
• Assign a chord already used by another action — conflict toast; Reset conflict when factory chord taken.
• ? on Keyboard shortcuts opens settings guide (verbosity high enough).
• Reload settings with a hand-edited "X" binding — dropped on load.
• Docs: review usage-settings.md#keyboard-shortcuts and links from usage.md / sketch hotkeys.

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Note

Low Risk
Localized to hotkey capture, load/save of gui.hotkeys, and documentation; no changes to core modeling or auth paths.

Overview
Tightens Settings → Keyboard shortcuts so remaps cannot break Move/Rotate or save unreadable chords.

Validation — Adds is_bindable_key (letters, digits, Space, and named keys that round-trip in JSON); punctuation and numpad are rejected at capture, in set_chord, and when loading gui.hotkeys. Unmodified X / Y / Z are now reserved in is_reserved_chord so axis constraints and rotation-axis pick stay available; invalid stored bindings are dropped on load.

UX — Unsupported, reserved, and conflict outcomes (including per-row Reset) call show_message for a status toast in addition to the inline table message. Settings adds a ? link (doc_urls::k_hotkeys) and updated help copy.

Docs — New Keyboard shortcuts section in usage-settings.md; usage.md / usage-sketch.md point there; CHANGELOG.md and src/doc/gui.md updated.

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