buzhash64: add FastCDC-style normalized chunking

[ThomasWaldmann]

Normalized chunking switches between a stricter and a looser cut mask around the target chunk size. This greatly tightens the chunk-size distribution (coefficient of variation ~0.9 -> ~0.3 in tests) and removes the dedup-hostile max-size-clamped chunks, with unchanged deduplication.

chunker-params for buzhash64 gains a required 6th field, nc_level:

buzhash64,chunk_min,chunk_max,chunk_mask,window_size,nc_level

Use nc_level=2 for the new default, nc_level=0 to disable (then behavior is byte-identical to the previous single-mask chunker).

buzhash (32bit) is untouched and stays bit-compatible with borg 1.x.

The mask transition point (normal_size) defaults to a principled formula (target minus the expected loose-phase tail) so the mean stays near the target; it can be tuned via the normal_size constructor arg.

scripts/chunker_bench.py: evidence harness used to measure chunk-size distribution, dedup ratio, throughput and shift-resilience.

Measurements (before = nc_level 0, after = nc_level 2; both at the default params buzhash64,19,23,21,4095; measured with scripts/chunker_bench.py):

5 GiB of incompressible data (~2000-2700 chunks, statistically stable):

before: CV 0.739, 49 max-size-clamped (8 MiB) chunks, 953 MB/s
after: CV 0.311, 0 max-size-clamped chunks, 1024 MB/s

Re-backup of a 2.5 GiB file after a few scattered single-byte edits (deduplication ratio; 0.5 = v2 fully deduplicated against v1, lower is better):

64 edits: before 0.5424 -> after 0.5235
320 edits: before 0.6791 -> after 0.6142

Normalized chunking deduplicates better after edits: removing the max-size-clamped chunks means a single-byte change invalidates much less data (about 36% less dedup overhead at 320 edits). Throughput was also consistently higher with nc_level=2 at this scale.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.98%. Comparing base (106dfba) to head (1a04046).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/helpers/parseformat.py	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9823      +/-   ##
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+ Coverage   84.94%   84.98%   +0.03%     
==========================================
  Files          92       92              
  Lines       15291    15295       +4     
  Branches     2296     2297       +1     
==========================================
+ Hits        12989    12998       +9     
+ Misses       1611     1604       -7     
- Partials      691      693       +2     

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