fix(read): preserve exact head semantics

[em0t]

Summary

• preserve exact head semantics in rewrites by routing head through an internal --head-lines mode instead of --max-lines
• rewrite both plain head file and numeric variants like head -20, head -n 12, and head --lines 7 to exact top-line slicing
• keep existing rtk read --max-lines summarizing behavior unchanged for direct rtk read usage
• add focused regression coverage for rewrite routing and exact head windowing

Minimal Cases

head

Fixture (input file contents)

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Raw

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Previous RTK

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This PR

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head -3

Fixture (input file contents)

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Raw

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Previous RTK

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This PR

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Testing

• cargo fmt --all
• cargo test test_rewrite_head -- --nocapture
• cargo test test_apply_line_window_head_lines -- --nocapture
• cargo test --all
• cargo clippy --all-targets (existing repo warnings only; none introduced by this PR)