fix(stock): skip ledger row on first-time pricing + include cost delta in notes

Code-review feedback from Task 2:
- Code-review Issue #1 (Important): the strict-equality guard fired on
  'before.cost_price = undefined' vs 'update.costPrice = 0' — exactly
  the population this feature targets (fresh catalogue rows the
  operator is pricing for the first time). No real cost movement was
  happening, but a spurious ledger row was getting written. Gate the
  ledger write on Number.isFinite(before.cost_price) so first-time
  pricing doesn't pollute audit history with a 0 -> 0 'movement'.
- Code-review Issue #4 (Minor, audit-readability): add the actual
  before -> after delta to the ledger notes so operators scrolling
  the ledger history see 'Gary edited cost 2.50 -> 3.20 from /stock'
  rather than the prior opaque 'Gary edited cost from /stock'.
- Added two regression tests: one for the first-time-pricing case,
  one pinning the notes delta format.

src/lib/server/stock-pricing-service.test.ts

@@ -143,4 +143,46 @@ describe("updateStockItemsPricing — stock_ledger audit on cost change", () =>
 
     expect(ledgerCreates).toEqual([]);
   });
+
+  it("does NOT write a stock_ledger row when prior cost_price is undefined (first-time pricing)", async () => {
+    const before = {
+      id: "abcdefghijklmno",
+      // cost_price intentionally missing — fresh catalogue row.
+      selling_price: 0,
+      markup: 0,
+    };
+    const updated = { ...before, cost_price: 2.5 };
+    const { pb, ledgerCreates } = stubPb(before, updated);
+
+    await updateStockItemsPricing({
+      pb: pb as never,
+      updates: [{ id: "abcdefghijklmno", costPrice: 2.5 }],
+      actor: { id: "useropid1234567", name: "Gary" },
+    } as UpdateStockItemsPricingInput);
+
+    expect(ledgerCreates).toEqual([]);
+  });
+
+  it("records an old → new delta in the notes string when costPrice changes", async () => {
+    const before = {
+      id: "abcdefghijklmno",
+      cost_price: 2.5,
+      selling_price: 3.25,
+      markup: 30,
+    };
+    const updated = { ...before, cost_price: 3.2 };
+    const { pb, ledgerCreates } = stubPb(before, updated);
+
+    await updateStockItemsPricing({
+      pb: pb as never,
+      updates: [{ id: "abcdefghijklmno", costPrice: 3.2 }],
+      actor: { id: "useropid1234567", name: "Gary" },
+    } as UpdateStockItemsPricingInput);
+
+    expect(ledgerCreates).toEqual([
+      expect.objectContaining({
+        notes: "Gary edited cost 2.5 → 3.2 from /stock",
+      }),
+    ]);
+  });
 });

src/lib/server/stock-pricing-service.ts

@@ -152,11 +152,14 @@ export async function updateStockItemsPricing(
     }
 
     // 3.5. Cost audit — write a stock_ledger row ONLY when costPrice was
-    //      supplied and differs from the existing cost_price. Side-effect:
+    //      supplied AND the prior cost was a real number AND it differs.
-    //      a failed ledger write aborts THIS row (we won't half-write the
+    //      First-time pricing (before.cost_price undefined) is NOT a
-    //      patch without recording the audit trail).
+    //      cost movement to audit — it's a field initialisation.
+    //      Side-effect: a failed ledger write aborts THIS row (we won't
+    //      half-write the patch without recording the audit trail).
     if (
       update.costPrice !== undefined &&
+      Number.isFinite(before.cost_price) &&
       before.cost_price !== update.costPrice
     ) {
       try {
@@ -166,7 +169,7 @@ export async function updateStockItemsPricing(
           unit_cost: update.costPrice,
           reason: "adjustment",
           reference_type: "manual",
-          notes: `${input.actor.name} edited cost from /stock`,
+          notes: `${input.actor.name} edited cost ${before.cost_price} → ${update.costPrice} from /stock`,
           created_by: input.actor.id,
         });
       } catch (err) {