app: rebuild the function index after an edit changes it

"even when I define functions with `p` I still get 'no functions: wrong
processor/base?'". Two bugs, and the second is the worse one.

The hint was LATCHED at load and only cleared on a reload, so it went on telling
you the image was described wrongly long after you'd proved otherwise. It is now
derived: the moment the index has a function, it stops being true.

But the index never had one. Nothing rebuilt _func_index after an edit, so `p`
gave you a function the rest of the app could not see — the names pane didn't
list it and Ctrl+N couldn't find it. The hint was just the visible symptom of
that.

_edit_done now reindexes when the edit changed which functions exist (`p` and
`u`; carving code doesn't, and a full walk after every `c` would be waste). It
uses its own worker rather than _load_functions(), which is the BOOT path — that
one clears the table, streams progress and auto-lands, which would yank the view
off the function you just made.

Verified on a blob with no functions: carve, `p`, and the status reads "created
function 0x4040–0x404c", the index reports 1, and the hint is gone.

Also: the reload confirmation said "1 functions". It counts now.

tests: +4 blob UI (30) — no functions and the hint says so, `p` creates one the
index can see, the stale hint is gone, the status names it. The Ctrl+L check
became wrong in the good way and now asserts the confirmation counts what would
be lost, since by then there IS something to lose. 209/0 scenarios, 13/0 thumb,
30/0 project UI.

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