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.

ui/dr-ui/src/develop/render.rs

@@ -547,7 +547,9 @@ impl DevelopSession {
     /// photograph nobody asked for, so every caller restores from a
     /// [`EditGraph::state`] taken first.
     pub(super) fn strip_adjustments(&mut self) {
-        Preset::default().apply(&mut self.graph, Scope::adjustments());
+        // The empty preset names no stock, so what it owes is nothing; the
+        // film is cleared through the session below.
+        let _ = Preset::default().apply(&mut self.graph, Scope::adjustments());
         *self.graph.masks_mut() = dr_pipeline::mask::MaskStack::new();
         *self.graph.spots_mut() = dr_pipeline::SpotSet::new();
         // Through the session rather than the graph: the baked tables live on