Add regex command implementation

tests/fixtures/regex/README.md

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+# Regex Command Fixtures
+
+These fixtures support contract and integration testing for the `renamer regex` command. Each
+subdirectory contains representative filenames used across preview, apply, conflict, and
+validation scenarios.
+
+- `baseline/` — ASCII word + digit combinations (e.g., `alpha-123.log`) used to validate basic
+  capture group substitution.
+- `unicode/` — Multilingual filenames to verify RE2 Unicode handling and ledger persistence.
+- `mixed/` — Build-style artifacts with underscores/dashes for automation-style rename flows.
+- `case-fold/` — Differing only by case to simulate case-insensitive duplicate conflicts.
+
+Tests should copy these directories to temporary working paths before mutation to keep fixtures
+idempotent.

tests/fixtures/regex/baseline/alpha-123.log

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+baseline fixture alpha-123

tests/fixtures/regex/baseline/beta-456.log

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+baseline fixture beta-456

tests/fixtures/regex/baseline/gamma-789.log

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+test fixture gamma-789

tests/fixtures/regex/case-fold/SAMPLE.txt

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+case fold fixture upper sample

tests/fixtures/regex/case-fold/Sample.txt

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+case fold fixture upper-lower sample

tests/fixtures/regex/case-fold/sample.txt

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+case fold fixture lower sample

tests/fixtures/regex/mixed/build_101_release.tar.gz

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+placeholder archive fixture 101

tests/fixtures/regex/mixed/build_102_hotfix.tar.gz

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+placeholder archive fixture 102

tests/fixtures/regex/mixed/feature-demo_2025-10-01.txt

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+feature demo placeholder file

tests/fixtures/regex/unicode/résumé-2025.md

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+Markdown résumé placeholder for Unicode accent coverage.

tests/fixtures/regex/unicode/项目-报告-01.txt

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+示例文件，用于验证Unicode处理。