Honor partial passthrough mode on commands

[x0Lazarus]

Closes #659.

A command tagged passthrough:"partial" currently stops flag parsing as soon as the command is selected, so command --flag value program forwards --flag value instead of parsing the inherited flag. Apply partial passthrough to the command's single positional argument so inherited flags are parsed until the first positional; arguments after that point are forwarded unchanged. Empty/default and explicit all modes retain their existing behavior.

The regression cases cover inherited long/short/equals flags, flags on either side of the positional boundary, unknown flags, --, a bare dash, empty input, and aliases. The README describes the command modes explicitly.

Validation on Windows:

• Full go test -count=1 ./... passes with Go 1.24.13 and Go 1.27.1.
• Repository-pinned golangci-lint 1.64.5 passes with the existing configuration using Go 1.24.13 and the Hermit-pinned Go 1.26.5.
• go vet -structtag=false ./... passes. Plain go vet ./... reports the same existing shorthand struct-tag diagnostics on the unchanged base and this patch; the project's lint configuration already excludes those diagnostics.

The complete hosted Linux/Windows matrix has not run locally.