SecretScanners relies on the undocumented standard library entry String._wordIndex(after:)

## What happens

`WordBreaks.isBoundary` in `Sources/UttrflowClipboard/SecretScanners.swift:403` calls `text._wordIndex(after:)`, which #458 used to find Unicode word boundaries in linear time. Underscored entries in the standard library are internal: they are not documented, not covered by source stability, and can change or disappear in any toolchain release.

## Why it matters

If a toolchain changes its behaviour, secret detection in the clipboard changes silently: a key, token or card number that should be masked could be shown. If it disappears, the build breaks with no obvious replacement.

## How to check

Search `Sources/` for `_wordIndex`. There is one use.

## Acceptance criteria

One of:

1. Replace it with a supported API that keeps the linear-time behaviour #458 measured (for example `NLTokenizer` or a boundary walk based on `Character` properties), with the scaling test in `SecretShapesScalingTests.swift` still passing; or
2. Keep it, and add a test that compares `WordBreaks` against `NSRegularExpression`'s `\b` on a table covering ASCII, digits, underscores, combining marks, emoji and CJK text, so a toolchain change fails loudly in `make verify`. The use gets a one-line comment pointing at that test.

In both cases `SecretDetectionTests` and `SecretShapesOracleTests` must pass unchanged.

Start at Sources/UttrflowClipboard/SecretScanners.swift:403 and search Sources/ for the single _wordIndex use. Compare the supported-API replacement or compatibility test against the acceptance criteria, including ASCII, digits, underscores, combining marks, emoji and CJK text. Run SecretShapesScalingTests.swift, SecretDetectionTests, SecretShapesOracleTests, and make verify; done means the required tests pass without changing the latter two suites.

Written by the indexing model from the issue text.