fix(base-io): stop excessive re-reads in LookaheadReader's single-char lookahead path

[deer]

LookaheadReader.prepare() (the no-arg overload used by consume(), peek(), available(), and follows()) delegated to prepare(DEFAULT_LOOKAHEAD), requesting a full 4096-character lookahead even for single-character operations.

Because the internal buffer is sized to exactly DEFAULT_LOOKAHEAD by default, this made the "not enough room" shuffle condition (index + desired > buffer.length) true again immediately after consuming just one character, forcing a fresh underlying read() call on essentially every single consume(). The fix changes the no-arg prepare() to request a desired of 1 instead, so the refill only triggers when the buffer is actually empty; the top-up read still fills as much of the buffer as is available, so bulk consumption via consume(int) is unaffected.