Support concatenated short flag syntax (-c100)

[mfridman]

Currently, short flag aliases only support -c 100 and -c=100 syntax. The concatenated form -c100 (no space or equals) fails with:

error: command "rewrap": flag provided but not defined: -c100

This is because short aliases are registered as regular flags in a flag.FlagSet via combineFlags(), and Go's flag.FlagSet.Parse() does not support the concatenated form.

Feasibility

This is doable with a small arg-rewriting pass in xflag.ParseToEnd. The FlagSet is already available there, so the approach would be:

1. Scan args for patterns like -X<value> where X is a known single-letter flag that expects a value
2. Expand -c100 into -c 100 before passing to flag.Parse()
3. Use the FlagSet to distinguish -c100 (short flag c with value 100) from -verbose (long flag named verbose) by checking if a single-char flag exists
4. Skip boolean flags since they don't take values (and -vn style combining is out of scope)

The scope is contained -- mostly changes to xflag/parse.go with maybe a small helper.