Cache the joined log buffer and skip it for single-line rules

[DigitalVeer]

This PR makes two changes on top of #1323:

1. The buffer caches the joined string. Push clears the cache.
2. CompilePattern marks the rules that can match only one line. Match tests these rules against the last line only.

This will help NPD act more reliably during log floods.

Benchmark

BenchmarkPushAndMatchAll (linux/arm64, Go 1.26):

	Time per line	Memory per line	Allocations per line
Before, buffer size 10	18484 ns	9221 B	26
After, buffer size 10	1659 ns	0 B	0
Before, buffer size 100	174067 ns	99118 B	28
After, buffer size 100	1687 ns	0 B	0

API change

• CompilePattern returns *Pattern instead of *regexp.Regexp.
• This removes String() from the LogBuffer interface as it is not used anymore.

[kubernetes-prow]

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