fix: avoid unnecessary allocation in preprocess

[eikopf]

Summary

• Change preprocess() return type from String to Cow<'_, str>
• Add a fast path that returns Cow::Borrowed when the input has no BOM, no CRLF sequences, and no shebang -- the common case for typical source files
• BOM removal uses a slice into the original input (still Borrowed), only CRLF normalization and shebang stripping allocate (Owned)
• No caller changes needed: Cow<str> derefs to &str transparently
• Add 4 new tests verifying the Borrowed/Owned behavior

Test plan

• All 19 preprocess unit tests pass (including 4 new Cow-variant tests)
• Full workspace test suite passes (570 tests across 12 suites)
• Fuzz target compiles cleanly (Cow<str> is compatible with existing assert_eq! comparisons)

Closes #19

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