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.

src/math_channel.hpp

@@ -58,6 +58,17 @@ struct MathNode {
   double hi = 1.0;
   double x = 0.0;       ///< the editor's box position, logical pixels.
   double y = 0.0;
+
+  /// Whether two nodes are the same node for the *engine's* purposes.
+  ///
+  /// `x`/`y` are the canvas's own and no op reads them, so a graph that only moved is not a change:
+  /// rebuilding for it replaces the buffer and drops its history -- and in a file view that means
+  /// re-reading the file -- to store two numbers nothing but the editor draws with.
+  bool operator==(const MathNode& other) const {
+    return op == other.op && source == other.source && in_a == other.in_a && in_b == other.in_b &&
+           gain == other.gain && offset == other.offset && fc == other.fc && q == other.q &&
+           lo == other.lo && hi == other.hi;
+  }
 };
 
 /// A derived channel: a graph of nodes and which node is the answer.
@@ -67,6 +78,11 @@ struct MathChannel {
   bool enabled = false;
   std::vector<MathNode> nodes;
   int output = -1;      ///< the node whose value is the channel's.
+
+  bool operator==(const MathChannel& other) const {
+    return name == other.name && unit == other.unit && enabled == other.enabled &&
+           output == other.output && nodes == other.nodes;
+  }
 };
 
 /// The per-channel runtime: the filter memory and the per-node scratch. Kept apart from the