fix: don't panic when capturing into struct types that implement Capture

[Pastalikek65]

Fixes #140.

Problem

A struct that implements Capture (or encoding.TextUnmarshaler) and is captured with @@ panicked at runtime:

panic: interface conversion: interface {} is parser.Box, not string

setField built the values []string argument from converted reflect values of the already-parsed struct, so v.Interface().(string) was a hard type assertion on a Box struct value.

Fix

• Capture is now fed from the raw token values, which is what the Capture contract actually expects (values []string): node.Parse collects token values in order, so values[0] is the captured token text.
• When the parsed field value is assignable to the field (e.g. a @@ struct that also implements Capture), it is pre-assigned so Pos/other already-parsed fields are kept.
• The encoding.TextUnmarshaler path was equally broken for struct types; it now unmarshals the joined token text (and keeps the parsed struct value).
• Slice captures keep passing plain string values unchanged.

Example (from the issue)

parser.ParseString("test", "abc::cdef.abc", &Boxes{})
// Boxes.Boxes.Val == "abc::cdef.abc"  (previously: panic)

Two new tests: TestStructCaptureViaExpression, TestStructCaptureWithTextUnmarshaler. The existing TestBoxedCapture/TestCaptureOnSliceElements continue to pass.