[alsoknownaswill]

Closes the remaining ways the install-hook scan could lose track of a written file or a shell variable:

• tilde forms (~+, ~0, ~+//x, a quoted or escaped tilde, an empty quote before a tilde) are read the way a shell reads them, in both the redirect/write tracker and the changed-file matcher
• ${V:-word} default words: quotes and backslashes are read as the shell reads them; unreadable ones hold
• := assignments inside any redirect target and the clobber ordering
• tests per class