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/library_ui/ratings_keywords.rs

@@ -592,11 +592,11 @@ fn collect_settings_writes(
                 // against one photograph and describes nothing on another.
                 // The target keeps whatever local adjustments it already had.
                 masks: None,
-                // A paste carries no film either, and for a plainer reason
-                // than the masks: a preset is a parameter map, and a stock is
-                // not a parameter. Pasting one would be pasting a choice the
-                // clipboard never captured.
-                film: None,
+                // TRACES: FR-DEV-3f
+                // The film travels where its sliders do: a stock is a look,
+                // not a fact about one frame, and a preset carrying one is
+                // how "Portra, printed" is applied to forty photographs.
+                film: preset.film_for(scope).map(|f| f.cloned()),
                 // Nor snapshots: they are states of the photograph they were
                 // taken on, and mean nothing on another.
                 snapshots: None,