fix(expansion): handle negative substring offset

[reubeno]

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Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

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Test Results

    2 files      9 suites   1m 32s ⏱️
  668 tests   668 ✅ 0 💤 0 ❌
1 322 runs  1 322 ✅ 0 💤 0 ❌

Results for commit 17b28e4.

♻️ This comment has been updated with latest results.

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Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	18.26 μs	18.37 μs	0.11 μs	🟠 +0.58%
eval_arithmetic	0.17 μs	0.17 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.60 μs	1.61 μs	0.01 μs	⚪ Unchanged
for_loop	21.97 μs	22.23 μs	0.26 μs	🟠 +1.20%
function_call	2.33 μs	2.36 μs	0.03 μs	⚪ Unchanged
instantiate_shell	57.61 μs	57.20 μs	-0.41 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	21713.35 μs	21631.83 μs	-81.52 μs	⚪ Unchanged
parse_bash_completion	1656.40 μs	1667.02 μs	10.63 μs	⚪ Unchanged
parse_sample_script	1.73 μs	1.77 μs	0.04 μs	🟠 +2.37%
run_echo_builtin_command	15.00 μs	15.19 μs	0.20 μs	⚪ Unchanged
run_one_external_command	2324.24 μs	2327.81 μs	3.56 μs	⚪ Unchanged
tokenize_sample_script	2.82 μs	2.84 μs	0.02 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/expansion.rs	🟢 96.17%	🟢 96.1%	🔴 -0.07%
Overall Coverage	🟢 75.67%	🟢 75.67%	⚪ 0%

Minimum allowed coverage is 70%, this run produced 75.67%