Fix Base64 histogram overflow

[carrerasdarren-cell]

Fixes #1509.

When Base64 mode adds a dedicated literal block type, both metablock paths can exceed the 256 histogram IDs representable by the context-map encoder:

• the high-quality path can cluster to 256 histograms and then append another;
• the greedy path can fill its histogram budget before appending one histogram per literal context.

Reserve the Base64 histogram slot while clustering, and only force the greedy Base64 split when all of its context histograms still fit. The regression test exercises the public streaming API at quality 9 and verifies a full encode/decode round trip.

Before this change, that test aborts with exit 134 and UBSan reports index 256 out of bounds for uint8_t[256] in MoveToFrontTransform. With the fix, it passes at qualities 5, 9, and 11 under UBSan.

Validation:

• Release CTest: 74/74 passed
• UBSan CTest: 74/74 passed
• Regression test compiles with -std=c89 -pedantic -Wall -Wextra -Werror

Thanks to @ylwango613 for the original report and reproducer.