perf: a file view re-reads the file only when a structural change really dropped it

Three tightenings, all measured with the reloads now reported on stderr.

`set_math_channels()` treats an identical list as no change at all. It rebuilt
unconditionally, and a rebuild replaces the buffer and drops its history -- which a file
view then has to re-read from disk. The comparison is the graphs', and it leaves the nodes'
`x`/`y` out deliberately: those are the canvas's own, no op reads them, and a graph that
merely *moved* should not cost the buffer.

`reload_file_view()` fires only when the store is actually empty. A change that really
happened leaves it so in a file view (the rebuild replaces it, and there is no producer to
refill it); one that found nothing to do leaves it full, and reading a 200 MB file again for
that is the waste. `ScopeModel::stored_slots()` is the one-line accessor it needed.

Every reload now says so, because a second read of a file is worth accounting for:

    [udpscope] reloaded <path> after a structural change: 402 samples back

Measured, `--open` a file:

    session with no graphs            0 reloads
    the math probe (3 real changes)   3 reloads, and all the readings stay equal
                                      (`math x2 402 samples vs 402 time slots`)

I had offered this as "one extra read at `--open` startup", and that was wrong: with no
session there is no `apply_math_channels()` call at all, so there was never an extra read
to remove. What the tightenings buy is that a no-op application is now free, and that a
reload cannot happen unless the buffer was really lost. Each *genuine* change still costs
one re-read -- a rebuild drops the history by design, and preserving it across a rebuild is
a model change rather than this one.

docs/claude/invariants-math-channels.md

@@ -42,7 +42,13 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   channel to an open waveform therefore emptied *every* channel -- `samples=0` on the sources too, which a user
   reads as "the plot is broken" -- and the same emptiness follows any structural change in a file view.
   `reload_file_view()` (in `ui.cpp`) re-reads the file after such a change, which is also what computes the
-  derived channels over it: the load's own pushes run the math once the set is in place. It is why `--open`
+  derived channels over it: the load's own pushes run the math once the set is in place. It fires only when the
+  buffer was *really* dropped -- `stored_slots() > 0` means the change found nothing to do -- and it says so
+  on stderr (`reloaded <path> after a structural change`), because a second read of a file is worth
+  accounting for. `set_math_channels()` in turn treats an identical list as no change at all, so a no-op
+  application (the session apply, a re-open) neither rebuilds nor drops nor re-reads; the comparison leaves the
+  nodes' `x`/`y` out on purpose, since those are the canvas's own and no op reads them -- a graph that only
+  *moved* is not a change, and rebuilding for it would drop the buffer to store two numbers. It is why `--open`
   works at all with a session that defines math channels, since `main()` loads the file *before* the UI (and
   so before the session's graphs) exists.
 - **Every path that fills a store must skip a derived channel, not just the packet path.**