Add -Fu (and -I) to the compiler's usage line

• Type: task — Track A (the usage string lives in compiler/**).
• Split from [[doc-n-fu-is-how-a-python-package-is-found]], resolved 2026-08-19: the prose is Track D's and is written; the usage line is not Track D's to edit, and leaving the remainder inside a resolved docs ticket would lose it.

What is missing

The compiler prints:

usage: pascal26/PXX [--debug] [--dump-ir] [-dNAME] [-uNAME] [-Mobjfpc]
       [--strict-overload] [--strict-operator] [--strict-case]
       [--strict-python] [--no-unhandled-handler] <src> [out]

-FuDIR is absent, and it is the flag that turns

error: import: no unit named mypkg and no shim mimic_mypkg

from "this feature does not exist" into "you did not say where to look". -I is absent for the same reason and belongs in the same line.

Note, measured 2026-08-19 against pin v364

-Fu takes the directory that contains the package, not the package directory: with pkgdir/mypkg/__init__.py, from mypkg import greet resolves under -Fu…/pkgdir and fails under -Fu…/pkgdir/mypkg. If the usage line gains a one-word gloss, that is the thing worth saying.

Gate

make compiler/pascal26 + tools/gate.sh quick. No behaviour change.