[Timing] Explain the Chord press window and fallback tradeoff

[malpern]

Part of #1203 and #888.

User problem

Chord Groups labels their only timing control simply Timeout. That does not say when timing starts, what must happen before expiry, or what happens when it expires. Qualitative presets describe speed without teaching the fallback tradeoff.

Approved vocabulary

Use Chord press window: the maximum time from the first participating key press until all chord keys are down. If the window expires, the keys fall back to their normal layer actions.

Scope

• Rename Timeout to Chord press window everywhere users encounter it.
• Add concise physical-action and expiry/fallback copy.
• Keep useful qualitative presets, but describe the precision versus fallback-latency tradeoff.
• Align visible copy, accessibility labels/values, tests, and user documentation.

Acceptance criteria

• No Chord Groups surface uses bare Timeout for this setting.
• Copy identifies first key press, last required key press, and normal-action fallback.
• Increasing/decreasing the value is described in felt terms.
• Preset names/descriptions agree with the actual millisecond values.
• Accessibility and copy-focused tests cover the control.
• The Chords user guide is updated.

Non-goals

• Changing chord recognition behavior.
• Redesigning chord conflict handling.