Let a preset name its film, and let a look reach only what it names

A preset could not choose a film stock. The stock is a choice of material
rather than a parameter, so `Preset` — a map of `op.param = value` — had
nowhere to hold it, and "Portra 400, printed" could not be saved, copied
or shipped as a look. Worse, the film node's own sliders *were*
parameters: a paste moved one stock's exposure and push onto whatever
stock the target was on, and left the target's tables baked from the
values it had just replaced.

A preset now carries a `FilmRef` beside its parameters. It travels under
whichever scope carries the film node, so the stock and its sliders are
never split, and by the replacement rule every other parameter follows:
applied at that scope, a preset without a film develops the target
without one. `Preset::apply` returns the `FilmRebake` it owes, as
`EditGraph::set_state` already did, because this crate cannot bake a
stock; the develop session pays it before recording the step, and the
batch paste writes the stock into each sidecar through `film_for`. The
library file spells it `film =` / `film_print =`, as a sidecar does, and
an older build keeps those lines as ones it does not understand.

`EditState` keeps the film in its own field only: the parameters it
captures leave it out, so one edit has one place to say which stock it
is on.

Second, a preset now has a reach. Replacement is right for a copy of a
whole edit — "make these match" — and wrong for a look: a stock-only
"Portra 400" applied that way would put the photograph's exposure, white
balance and noise reduction back to default. `Reach::Named` replaces only
the operations a preset names (whole operations, so a look that sets the
blacks resets the whites beside them) and the film only if it names one.
Saved edits and the clipboard keep `Reach::Whole`; the line `reach =
named` is written only for the other, so existing libraries write the
same bytes.

core/dr-pipeline/src/graph.rs

@@ -623,7 +623,9 @@ impl EditGraph {
         } = self;
 
         EditState {
-            params: Preset::capture(self),
+            // Without the film: it has a field of its own below, and one
+            // edit must not have two places to disagree about its stock.
+            params: Preset::capture_params(self),
             // A refcount bump. See `EditState::masks` for why that matters on
             // a path called once a frame.
             masks: Arc::clone(masks),
@@ -663,7 +665,11 @@ impl EditGraph {
         // At full scope. `Scope` is a question about what a paste carries
         // *between* photographs; this is one photograph's own edit being put
         // back, so there is nothing to leave behind.
-        params.apply(self, Scope::everything());
+        //
+        // What the parameters say about the film is discarded: the stock is
+        // `film`'s to decide, below, and clearing it is what happens there
+        // either way.
+        let _ = params.apply(self, Scope::everything());
 
         self.masks = Arc::clone(masks);
         self.spots = spots.clone();