[runtime/utils/buffer/append] use Blob::write_at_sync

[andresilva]

This PR updates buffer::paged::Append to use Blob::write_at_sync when a flush can be persisted with a single physical write. When a flush is split around a protected CRC slot, or when a prior plain write or resize still needs a durability barrier, it falls back to plain writes followed by a single full sync. It now tracks whether prior blob mutations still need a full sync so sync() can skip work when there is nothing pending. Resize/shrink handling was adjusted so real physical resizes are made durable correctly, while logical-only partial shrinks can keep using range sync for their CRC updates.

Depends on #3852.

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Status	Name	Latest Commit	Updated (UTC)
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Benchmark results

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✅ PASSED: No benchmark exceeded the regression threshold.

Benchmark comparison table

Benchmark	Baseline (main)	Current	Delta	Threshold	Status
qmdb::merkleize/variant=any::unordered::fixed::mmr keys=10000 ch=false sync=false	1.577 ms	1.624 ms	+3.00%	10.00%	✅ PASS
qmdb::merkleize/variant=current::ordered::fixed::mmb chunk=256 keys=10000 ch=true sync=false	3.059 ms	3.218 ms	+5.18%	10.00%	✅ PASS

Baseline commit(s): c1cd5a32740d

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[codecov]

Codecov Report

❌ Patch coverage is 95.89041% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.77%. Comparing base (3c08216) to head (837ad2f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
runtime/src/utils/buffer/paged/append.rs	95.45%	11 Missing and 7 partials ⚠️

@@            Coverage Diff             @@
##             main    #3857      +/-   ##
==========================================
+ Coverage   95.75%   95.77%   +0.02%     
==========================================
  Files         486      486              
  Lines      200864   201251     +387     
  Branches     4861     4876      +15     
==========================================
+ Hits       192341   192756     +415     
+ Misses       6891     6860      -31     
- Partials     1632     1635       +3     

Files with missing lines	Coverage Δ
runtime/src/utils/buffer/mod.rs	97.30% <100.00%> (+1.95%)	⬆️
storage/src/journal/segmented/variable.rs	93.94% <100.00%> (+0.01%)	⬆️
runtime/src/utils/buffer/paged/append.rs	97.71% <95.45%> (+0.85%)	⬆️

... and 8 files with indirect coverage changes

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