[sameerihs]

Treat = in -s= as the value separator even when the value is empty.
The current length check instead gives a string flag "="; optional-value
flags use their fallback and then fail on an unknown = shorthand.

Apply the same boundary correction to unknown shorthand handling so an
ignored -u= does not consume the next positional argument. Regressions cover
Parse, ParseAll, clusters, optional values, unknown flags and empty boolean
errors, with controls for non-empty values and a literal equals value.

Validation on Go 1.27.3/macOS arm64:

• Eleven regression cases fail before the fix; all 19 cases pass afterward.
• Full race suite, vet, build, repository-configured lint and formatting pass.
• An additional 44 public-API review probes pass.
• Linux/386 tests cross-compile; the binary was not executed locally.

Go 1.21.13 aborts before running tests on this Mac with
missing LC_UUID load command; pristine upstream fails identically. The
remaining Linux/Go-version CI matrix still needs to run.

This draft awaits confirmation of the contributor-agreement status.

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