vers: HighestSatisfying helper for resolver-style picks

[andrew]

Summary

Adds `vers.HighestSatisfying(versions []string, constraint, scheme string) (string, error)`. Common shape of a package-manager resolver:

```go
parsed, _ := vers.ParseNative(constraint, scheme)
var best string
for _, v := range versions {
if !parsed.Contains(v) {
continue
}
if best == "" || vers.CompareWithScheme(v, best, scheme) > 0 {
best = v
}
}
```

Becomes:

```go
best, err := vers.HighestSatisfying(versions, constraint, scheme)
```

Behaviour

• Empty `scheme` parses `constraint` as a vers URI; non-empty parses as native syntax (same dispatch as `Satisfies`).
• Versions that fail to parse are skipped — registry data is messy in practice; a single bad entry shouldn't abort the walk.
• Returns `("", nil)` when no version satisfies, distinct from `("", err)` when the constraint itself fails to parse.
• Ordering uses `CompareWithScheme` so scheme-specific precedence rules (npm pre-release ordering, etc.) apply.

Tests

Five: npm constraint coverage (caret, tilde, range, intersection, no-match), order independence on the input slice, garbage-version skipping, vers-URI mode via empty scheme, and the empty-input case.