Make selectors serialisable

A smart collection stores its selector as JSON in collections.selector_json,
and cache rules carry one across a sync, so the predicate language has to
round-trip. Derive-only: no serde machinery leaks into the rest of core/.

FlagState gains Default = Unflagged, which is what an image is before anyone
has looked at it. Any other default would assert a judgement the
photographer never made.

Assisted-by: LLM

core/dr-types/Cargo.toml

@@ -7,3 +7,7 @@ license.workspace = true
 
 [dependencies]
 thiserror.workspace = true
+# Selectors are stored as JSON in `collections.selector_json` and travel in
+# sync'd cache rules, so the predicate language has to serialise. Derive-only:
+# no serde machinery leaks into the rest of `core/`.
+serde.workspace = true

core/dr-types/src/lib.rs

@@ -14,7 +14,8 @@ pub use selector::{ColourLabel, DateSelector, FlagState, Selector, Tier};
 
 /// Identifies a granted library location — a directory on Linux, a persisted
 /// document tree on Android.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, serde::Serialize, serde::Deserialize)]
+#[serde(transparent)]
 pub struct RootId(pub u64);
 
 /// Identifies an image within a catalog.
@@ -26,7 +27,8 @@ pub struct ImageId(pub u64);
 pub struct VersionId(pub u64);
 
 /// Identifies a user-defined collection.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, serde::Serialize, serde::Deserialize)]
+#[serde(transparent)]
 pub struct CollectionId(pub u64);
 
 /// Identifies a folder within a root.
@@ -244,7 +246,7 @@ impl FormatFilter {
 ///
 /// Surfaced in the UI so a user always knows what they have — the failure
 /// Lightroom makes by showing an original's filename beside a 2560px proxy.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, serde::Serialize, serde::Deserialize)]
 pub enum Availability {
     /// Catalogued, nothing cached.
     MetadataOnly,

core/dr-types/src/selector.rs

@@ -20,7 +20,7 @@ use crate::{Availability, CollectionId, RootId};
 /// Compiles to indexed SQL in `dr-catalog`; evaluated against catalog state
 /// for cache rules. Deliberately data — no closures, so it serialises into a
 /// saved collection or a sync'd cache rule.
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum Selector {
     /// Everything. The empty filter, and the implicit default cache rule.
     All,
@@ -103,7 +103,7 @@ impl Selector {
 }
 
 /// A date constraint, absolute or moving.
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum DateSelector {
     /// UTC seconds, half-open: `from <= t < to`.
     Between { from: i64, to: i64 },
@@ -115,7 +115,7 @@ pub enum DateSelector {
 }
 
 /// Colour labels, matching the conventional set.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, serde::Serialize, serde::Deserialize)]
 pub enum ColourLabel {
     Red,
     Yellow,
@@ -125,9 +125,17 @@ pub enum ColourLabel {
 }
 
 /// The pick/reject axis, independent of star rating.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+///
+/// `Unflagged` is the [`Default`] because it is what an image *is* before
+/// anyone has looked at it — an unjudged frame, not a rejected one. Any other
+/// default would silently assert a judgement the photographer never made.
+#[derive(
+    Debug, Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash,
+    serde::Serialize, serde::Deserialize,
+)]
 pub enum FlagState {
     /// Not yet judged. What "filter to unjudged" selects (FR-CULL-4).
+    #[default]
     Unflagged,
     Pick,
     Reject,
@@ -136,7 +144,7 @@ pub enum FlagState {
 /// How much of an image a cache rule asks to keep locally.
 ///
 /// Ordered so the most generous matching rule wins (ARCH §9.3).
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, serde::Serialize, serde::Deserialize)]
 pub enum Tier {
     /// Catalog rows and sidecars only. Never evicted — authoritative and tiny.
     Metadata,