improvise-km71: serde back in improvise-formula for FormulaId alone

tt22 removed the formula crate's serde dependency with the legacy JSON loader,
keeping serde only where a type crosses the wire. FormulaId crosses the wire, so
it is the third such type. No derive goes back on the formula AST.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

Cargo.lock

@@ -897,6 +897,7 @@ dependencies = [
  "pest_derive",
  "pest_meta",
  "proptest",
+ "serde",
 ]
 
 [[package]]

context/code/formula-and-io.md

@@ -488,8 +488,12 @@ The serde derives that existed only for that path went with it: `Workbook`,
 `DataStore`'s manual impls and the formula AST. So did indexmap's `serde` feature
 in `improvise-core`, `improvise-io` and the root crate, and `improvise-formula`'s
 serde dependency. Each was removed and the whole workspace rebuilt, so the
-compiler named every other consumer. Two remain, and both are the wire:
+compiler named every other consumer. Two remained, and both are the wire:
 `Axis` (whose `rename_all = "lowercase"` is its wire spelling) and `PageAxis`.
+A third arrived with improvise-km71 by the same rule: `FormulaId`, which crosses
+the wire in `ModelCommand::ReplaceFormula`, `FormulaSlot` and `FormulaEntry`.
+So `improvise-formula` depends on serde again, for that one type. No derive went
+back on the formula AST.
 `CellValue` was never a candidate, since it is carried on the wire too. JSON
 *import* (`StartImportWizard`, `ImportJsonHeadless`) is a separate feature and is
 untouched.

crates/improvise-formula/Cargo.toml

@@ -10,6 +10,8 @@ repository = "https://github.com/fiddlerwoaroof/improvise"
 anyhow = "1"
 pest = "2.9.2"
 pest_derive = "2.9.2"
+# For FormulaId alone, which crosses the wire (improvise-km71); no AST derive.
+serde = { version = "1", features = ["derive"] }
 
 [dev-dependencies]
 pest_meta = "2.9.2"

crates/improvise-formula/src/ast.rs

@@ -101,7 +101,7 @@ impl Formula {
 
 /// Which formula an edit means, opaque so its content can change in one place:
 /// today a target and its category — `context/code/core.md` §18.
-#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, serde::Serialize, serde::Deserialize)]
 pub struct FormulaId {
     target: String,
     target_category: String,

harness/opfs-contract/Cargo.lock

@@ -196,6 +196,7 @@ dependencies = [
  "anyhow",
  "pest",
  "pest_derive",
+ "serde",
 ]
 
 [[package]]