docs(guide): Events, commands & keymaps page

[blendsdk]

Part of #143 — the Guide expansion. Group Core concepts. Sibling of the existing
Keyboard & clipboard page.

Purpose & audience

Teach the input model end to end: what a key/mouse event looks like, how the event loop routes it,
how commands decouple "what the user did" from "what happens", and how to rebind keys with a keymap.

The gap today

Keyboard & clipboard covers only the clipboard slice. Nothing explains the shape of an event, the
command indirection, custom keymaps, mouse events, or bracketed paste. A misspelled command: 'sve'
is silently dropped (issue #141, footgun 5) — the reader has no way to know why their button is dead.

Outline

1. The event loop: createEventLoop routes keyboard + mouse to views and drives one render per tick;
   host-agnostic (you supply the input source).
2. Event shapes: key events (name + modifiers), mouse events (button/drag/wheel), paste events
   (bracketed paste, capped at PASTE_CAP_BYTES).
3. Commands: Commands registry + CommandEvent; a button emits a command, a handler consumes it;
   an unregistered command name is dropped (the silent-'sve' footgun) — how to catch it.
4. Keymaps: createKeymap / buildKeymap, KEY_NAMES, and ClipboardKeys ('both'/'modern'/
   'classic'/'none'); a user keymap always wins over defaults on conflict.
5. ESC_TIMEOUT_MS and why a lone ESC waits (distinguishing ESC from an escape sequence).

Code examples to write

• A custom keymap binding an app command to a chord.
• Emitting and handling a custom command (button → handler → visible effect).
• Freeing keys for your own bindings with clipboardKeys: 'none'.

Live examples

• Reuse: the existing apps/editor (Keyboard & clipboard page) demonstrates live key handling —
  free to reference.
• Build new: optional guide/keymap echoing the decoded name of whatever key you press; strong
  teaching value for "what does an event look like", build if budget allows.

Source pointers

• packages/core/src/engine/input/ — decoder.ts, events.ts, keys.ts, keymap.ts, mouse.ts,
  paste.ts; ESC_TIMEOUT_MS / PASTE_CAP_BYTES / KEY_NAMES exported from input/index.ts.
• packages/ui/src/event/ — createEventLoop, buildKeymap, CommandEvent, ClipboardKeys.
• packages/ui/src/event/commands.ts:59-60 (unregistered command dropped).
• Memory global-clipboard-work (default keymap + clipboardKeys design).

Constraints

• No .layout = (snippet-drift). Reuse the existing clipboard prose; don't duplicate it — link.

Acceptance criteria

• Event shapes (key/mouse/paste), the loop, commands, and keymaps all covered with runnable code.
• The silent-dropped-command footgun documented with how to detect it.
• Clearly scoped against the sibling Keyboard & clipboard page (no duplication).
• New example (if built) passes gates; build clean.