Prevent overflow in file-lines range normalization

[cuishuang]

Fixes #6992

Range::adjacent_to previously added one directly to user-controlled --file-lines upper bounds. An upper bound of usize::MAX could therefore panic when overflow checks were enabled or wrap in release builds.

The wrapping case could also incorrectly merge non-adjacent ranges because the JSON parser accepted line zero, despite --file-lines being documented as 1-based.

This change:

• uses checked_add for range adjacency checks;
• rejects zero-based and reversed ranges parsed from --file-lines;
• keeps usize::MAX as a valid upper bound;
• adds regression tests for maximum bounds and invalid ranges.