fix: satisfy strict null checks in spk/math/visibility

src/math/index.ts

@@ -130,7 +130,7 @@ export function rotZ(theta: number): Mat3 {
 export function chebyshevEval(coeffs: Float64Array, x: number): number {
   const n = coeffs.length
   if (n === 0) return 0
-  if (n === 1) return coeffs[0]
+  if (n === 1) return coeffs[0]! // Float64Array length checked above; index 0 is valid
 
   // Double-x for Clenshaw efficiency
   const x2 = 2 * x
@@ -138,12 +138,12 @@ export function chebyshevEval(coeffs: Float64Array, x: number): number {
   let b1 = 0
 
   for (let k = n - 1; k >= 1; k--) {
-    const b0 = coeffs[k] + x2 * b1 - b2
+    const b0 = coeffs[k]! + x2 * b1 - b2 // k in [1, n-1]; all valid indices
     b2 = b1
     b1 = b0
   }
 
-  return coeffs[0] + x * b1 - b2
+  return coeffs[0]! + x * b1 - b2 // index 0 valid; n >= 2 at this point
 }
 
 /**
@@ -162,7 +162,7 @@ export function chebyshevEvalWithDerivative(
 ): [number, number] {
   const n = coeffs.length
   if (n === 0) return [0, 0]
-  if (n === 1) return [coeffs[0], 0]
+  if (n === 1) return [coeffs[0]!, 0] // length checked; index 0 valid
 
   const x2 = 2 * x
   let b2 = 0
@@ -171,7 +171,7 @@ export function chebyshevEvalWithDerivative(
   let db1 = 0
 
   for (let k = n - 1; k >= 1; k--) {
-    const b0 = coeffs[k] + x2 * b1 - b2
+    const b0 = coeffs[k]! + x2 * b1 - b2 // k in [1, n-1]; all valid indices
     const db0 = 2 * b1 + x2 * db1 - db2
     b2 = b1
     b1 = b0
@@ -179,7 +179,7 @@ export function chebyshevEvalWithDerivative(
     db1 = db0
   }
 
-  const value = coeffs[0] + x * b1 - b2
+  const value = coeffs[0]! + x * b1 - b2 // index 0 valid; n >= 2 at this point
   const dvalue = b1 + x * db1 - db2
   // Scale derivative from normalized domain back to seconds
   return [value, dvalue / radius]
@@ -300,7 +300,7 @@ export function findRoots(f: (t: number) => number, a: number, b: number, steps
       const root = brentRoot(f, tPrev, t, 1e-9, 64)
       if (root !== null) {
         // Deduplicate roots that are too close together
-        if (roots.length === 0 || Math.abs(root - roots[roots.length - 1]) > 1e-6) {
+        if (roots.length === 0 || Math.abs(root - roots[roots.length - 1]!) > 1e-6) { // length > 0 checked via ||
           roots.push(root)
         }
       }

src/spk/index.ts

@@ -386,7 +386,7 @@ export function parseLsk(text: string): ReadonlyArray<readonly [number, number]>
   const match = text.match(/DELTET\/DELTA_AT\s*=\s*\(\s*([\s\S]*?)\)/m)
   if (!match) return results
 
-  const block = match[1]
+  const block = match[1]! // regex has one capture group; match[1] is present when match succeeds
   const months: Record<string, number> = {
     JAN: 1,
     FEB: 2,
@@ -406,10 +406,11 @@ export function parseLsk(text: string): ReadonlyArray<readonly [number, number]>
   let m: RegExpExecArray | null
 
   while ((m = pairRe.exec(block)) !== null) {
-    const deltaAT = parseFloat(m[1])
-    const year = parseInt(m[2])
-    const month = months[m[3]] ?? 1
-    const day = parseInt(m[4])
+    // m[1]..m[4] are capture groups; regex guarantees they are present when exec matches
+    const deltaAT = parseFloat(m[1]!)
+    const year = parseInt(m[2]!)
+    const month = months[m[3]!] ?? 1
+    const day = parseInt(m[4]!)
 
     // Gregorian to JD (noon = integer JD)
     const a = Math.floor((14 - month) / 12)

src/visibility/index.ts

@@ -285,5 +285,5 @@ function azimuthToCardinal(az: number): string {
     'NNW',
   ]
   const idx = Math.round(az / 22.5) % 16
-  return dirs[(idx + 16) % 16]
+  return dirs[(idx + 16) % 16]! // dirs has 16 elements; (idx+16)%16 is always 0..15
 }