sed: fix clippy cast_possible_truncation errors

[sylvestre]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.27%. Comparing base (d28a1f8) to head (98df87f).

Files with missing lines	Patch %	Lines
src/sed/processor.rs	0.00%	4 Missing ⚠️
src/sed/fast_io.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
- Coverage   83.29%   83.27%   -0.03%     
==========================================
  Files          13       13              
  Lines        6987     6994       +7     
  Branches      396      397       +1     
==========================================
+ Hits         5820     5824       +4     
- Misses       1164     1167       +3     
  Partials        3        3              

Flag	Coverage Δ
macos_latest	83.91% <66.66%> (-0.03%)	⬇️
ubuntu_latest	84.12% <70.58%> (-0.03%)	⬇️
windows_latest	0.00% <0.00%> (ø)

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[codspeed-hq]

Merging this PR will regress 1 benchmark

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 9 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	number_fix	1.6 s	1.7 s	-5.11%
⚡	access_log_subst	2.5 s	2.3 s	+7.71%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing sylvestre:clippy-cast-truncation (98df87f) with main (d28a1f8)}