Add single-slot caption mode

[BradenHartsell]

Implements --single-slot-captions for #344.

What changed:

• Adds a boolean caption setting and command-line flag.
• When enabled, a newly-triggered caption deletes any visible active captions before being displayed at the normal caption position.
• Updates README, man page, and ChangeLog.

Verification:

• git diff --check passed locally.
• I could not run the C++ test suite in this Windows environment because make and g++ are not installed.

Bounty note:

• If this qualifies for the issue bounty, I am happy to coordinate payout details privately after acceptance and follow your preferred process.