Document record once behavior for loop duration and fade handling

[Copilot]

Addresses #49: Documents how "record once" works regarding duration and fade handling.

Key findings:

• Record once uses existing loop start/end points, not a fixed duration
• Crossfading subheads handle fade at loop boundaries automatically—no need to record past the loop end

Changes:

• readme.md: Added "Record Once Behavior" section under Playing/Recording
• ReadWriteHead.h/.cpp: Added code comments documenting the record-once mechanism

The crossfade architecture with dual subheads means fade regions are handled inherently during loop transitions, eliminating the need for additional recording past loop endpoints.

Original prompt

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This section details on the original issue you should resolve

<issue_title>examine record once</issue_title>
<issue_description>Examine record once. Does it set the number of seconds? Does it record past the end of the record point to have stuff for fade?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes examine record once #10

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