Warn Windows users about backslash path separators in glob patterns - #1539

Reviewed all six changed files and #1463. This implements the issue's warning-only option: matching semantics and pattern contents remain unchanged. The issue search also found the later #1550 alternative; that overlap does not create a dependency for this earlier implementation.

The CLI path warns before appending its patterns, while rule loading warns only for the selected include/exclude layer. Both use stderr, leaving machine-readable stdout alone. The helper consumes trusted escape pairs together, detects dangling backslashes, and uses %q for user-controlled pattern text. No blocking defect found in the warning-only behavior.

I independently ran an isolated Go harness containing the exact new classifier/output algorithm: all 16 supplied classifier cases passed; the Windows branch emitted exactly three safely quoted warning lines for the mixed fixture; the Linux branch emitted none. This does not exercise doublestar itself or substitute for the repository/Windows integration suite.

One non-blocking test improvement: TestBuildFileFilter_WarnsOnlySelectedLayer searches for the raw backslash pattern, while the warning quotes it with doubled backslashes. Assert an empty buffer, or search for strconv.Quote(pattern), so that test would actually fail if an inactive layer were accidentally warned. The implementation currently returns after the selected layer, so this is test strength rather than a production defect.

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