diffguard-testkit/diff_builder.rs: usize→u32 truncation casts — should use try_from

[EffortlessSteven]

clippy::cast_lossless / clippy::cast_possible_truncation: is cast to with , which silently truncates on 64-bit targets.

Locations (all in ):

• Line 464:
• Line 481: (two casts)
• Line 519:
• Line 538:

Fix: use and handle the error, or with a comment explaining why truncation is acceptable for the test fixture's bounded inputs (MAX_LINES_PER_HUNK = 20 per arb.rs).

Filed separately from the same issues in diffguard-analytics (issue #1577) since those are in production lib code.