fix(labs): never open a small lab value as zero in the edit sheet

The edit seed rounded every value to two decimals, so a TSH of 0.004 opened
as 0. Values at or above 1 keep the two-decimal display precision; below 1
the seed keeps three significant figures, written without exponent
notation.

src/lib/labs/__tests__/value-update.test.ts

@@ -12,6 +12,18 @@ describe("valueEditSeed", () => {
     expect(valueEditSeed(110)).toBe("110");
     expect(valueEditSeed(4.567)).toBe("4.57");
   });
+
+  it("never opens a small nonzero value as zero", () => {
+    expect(valueEditSeed(0.004)).toBe("0.004");
+    expect(valueEditSeed(0.0123456)).toBe("0.0123");
+    expect(valueEditSeed(0.87)).toBe("0.87");
+    expect(valueEditSeed(0.123456)).toBe("0.123");
+    expect(valueEditSeed(-0.004)).toBe("-0.004");
+    expect(valueEditSeed(0.0000004)).toBe("0.0000004");
+    for (const v of [0.004, 0.0004, 0.00004, 1e-9]) {
+      expect(Number(valueEditSeed(v))).not.toBe(0);
+    }
+  });
 });
 
 describe("resolveValueForUpdate", () => {

src/lib/labs/value-update.ts

@@ -1,18 +1,33 @@
-import { formatLabValue } from "./format-value";
-
 /**
  * The numeric value of a lab reading in the edit sheet.
  *
  * A converted reading is stored at full precision (5.6 mmol/L of glucose is
  * 100.90191999999999 mg/dL), and the sheet must not open with that tail. The
- * field starts from the value as every lab surface shows it: whole numbers
- * bare, at most two decimals. Written with a dot, which the sheet's parser
- * reads, rather than in the reader's locale, which an input cannot round-trip.
+ * field starts from the value at the precision every lab surface shows it:
+ * whole numbers bare, two decimals. Two decimals carry too little of a small
+ * value, though (a TSH of 0.004 would open as "0"), so below 1 the field keeps
+ * three significant figures instead. Written with a dot, which the sheet's
+ * parser reads, rather than in the reader's locale, which an input cannot
+ * round-trip, and never in exponent notation.
  */
 export function valueEditSeed(value: number): string {
-  return formatLabValue(value, String);
+  if (Number.isInteger(value)) return String(value);
+  const magnitude = Math.floor(Math.log10(Math.abs(value)));
+  const decimals = Math.min(
+    MAX_SEED_DECIMALS,
+    Math.max(DISPLAY_DECIMALS, SEED_SIGNIFICANT_FIGURES - 1 - magnitude),
+  );
+  return value
+    .toFixed(decimals)
+    .replace(/(\.\d*?)0+$/, "$1")
+    .replace(/\.$/, "");
 }
 
+const DISPLAY_DECIMALS = 2;
+const SEED_SIGNIFICANT_FIGURES = 3;
+/** `toFixed` accepts up to 100; no lab value needs more than this. */
+const MAX_SEED_DECIMALS = 20;
+
 /**
  * What the edit PUT sends for the value. A field the person did not touch is
  * omitted (`undefined`), so the stored full-precision number stays as it is and