Give an edit one complete state, and make omitting part of it a compile error

An edit used to be a bag of scalars. That stopped being true when the mask
stack and the film stock arrived, both deliberately held apart from `ops`
because a layer is not a scalar and a stock is not a scalar — and nothing
announced the change. What happened instead is that three routines each
captured "the edit" and each captured a different subset of it.

`EditState` is all of it: the parameter map, the masks, the stock. What keeps
it complete is not a comment. `EditGraph::state` destructures the graph
exhaustively, `EditGraph::set_state` destructures the state exhaustively, and
the fields are public so every construction site is a struct literal naming
all of them. Adding a fifth kind of graph state — FR-DEV-8's spot removal is
the one already asked for — fails to compile until somebody has decided
whether an undo has to put it back. Verified both ways round by adding a field
to each type and watching five call sites refuse to build.

A compiler error rather than a runtime check, because the failure being
prevented is silence: the missing halves produced no panic, no warning and no
failing test.

The stack is now shared rather than owned, and that is about the drag path
rather than memory: `state` runs on every parameter change, which during a
drag is once a frame, and deep-copying a painted brush sixty times a second to
record an exposure move would be a cost paid for nothing. `masks_mut` is the
one door a stack is modified through, so it clones on write.

`FilmRebake` is the one thing a caller is still owed. Restoring a stock always
needed the profile database this crate does not link (ARCH §6.5a); it was a
comment before, and it is a `#[must_use]` return value now.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-pipeline/src/lib.rs

@@ -43,6 +43,7 @@ pub mod ops;
 pub mod preset;
 pub mod sidecar;
 pub mod spot;
+pub mod state;
 
 pub use descriptor::{
     Attribute, Facet, LocalizedKey, OpDescriptor, OpId, ParamDescriptor, ParamId, ParamKind,
@@ -62,6 +63,7 @@ pub use operation::{
 pub use preset::{Preset, Scope};
 pub use sidecar::{Sidecar, Version};
 pub use spot::{Spot, SpotMode, SpotSet};
+pub use state::{EditState, FilmRebake, FilmRef};
 
 #[cfg(test)]
 mod tests {