Add `MultiArgMatcher` for greedy flag matching and
`MultiPickableWithBoundary` trait with `VecUntil<T>` wrapper for bounded
multi-value collection.

Introduce `BoundaryCheck` trait to allow positional multi-value
parameters to stop consuming arguments when a non-matching value is
encountered.

Extract positional argument matching from ArgMatcher into a dedicated
matcher that respects the `--` terminator and skips flag-like tokens
before it.  Add test suite covering basic, after-`--`, and Windows-style
behaviour.

Implement `Pickable` for `String` supporting both positional and named
flags, and generalize value matching to use style-specific separators

Deduplicate the `seek_end_of_options` function by moving it from both
`arg_matcher` and `flag_matcher` into a single shared implementation in
utils. Add `make_masked` and `make_args` test helpers to the test crate

Implement `on_match_one` and `on_match_all` for `ArgMatcher`,
supporting named flags (with eq-mode and short flags), positional
arguments, end-of-options marker (`--`), and case-insensitive matching

Add a new `Flag` enum in `mingling_picker::value` that explicitly
distinguishes between an absent flag (`Inactive`) and a present flag
(`Active`), along with its `Pickable` implementation.  Rename internal
fields from `flag_*` to `arg_*` for consistency, update the default
naming case to `Kebab`, and enable the `mingling_support` feature in
workspace settings.