Cache the indentation strings of the output lines (google-java-format #1466)

[abashev]

Port of google/google-java-format#1466 (Liam Miller-Cushon, author kept). Every output line's indentation was a fresh " ".repeat(indent); JavaOutput.spaces(int) now hands out one String per width up to 100 spaces, built on first use. Wider widths are built as before, zero or less gives the empty string. Upstream's second half, Style.indentString delegating to the cache, has no counterpart here: the fork's Style knows no tabs, and JavaOutput.indent is the only caller.

TDD

JavaOutputTest was written first and did not compile: the same instance for the same width, the empty string at zero and below, and a width beyond the cache.

What it buys

Measured with a small harness (ThreadMXBean.getThreadAllocatedBytes on one thread, the 135 java.util files of JDK 21, six warm-up and six measured passes, three JVMs each):

	allocated per pass	per output line	time per pass
main	1,980–1,981 MB	17,761–17,772 B	828–878 ms
this branch	1,976–1,978 MB	17,728–17,746 B	821–843 ms

About 30 bytes per output line, 0.2 % of what formatting allocates, and the time does not move. It is upstream alignment more than a measurable win; merging decides whether that is worth the twenty lines.

Checks

• :open-java-format:test on JDK 21: 1600 tests, 0 failures.
• The JDK 21 sources (15,747 files), formatted with main's jar and this branch's jar, are identical.

[abashev]

Closed unmerged: the measured gain is 0.2 % of what formatting allocates, which does not pay for the extra code. The regex port, #104, stays.