Add a 10-cell log2-bucketed histogram showing file-size
distribution across the directory. Each cell is the count of
files in a bucket (1B..1KiB, 1KiB..2KiB, ... >=512GiB) scaled to
a 0..8 block-char level. Rendered in accent colour next to
the existing per-file size bar.
- size_histogram() returns [u64; 10] counts per bucket
- Uses u64 leading_zeros trick for O(1) log2 size
- Skips directories and the '..' entry
- Directories are excluded from the histogram
- Renders after the cursor file's size bar
Added 1 unit test verifying bucket placement for 2B and 16KiB.
Tests: 1400 pass (was 1399), zero warnings.
vasilito
e7d5dad0aa