refactor(bigint): read each from_octets limb with a u32be pattern

bigint/bigint_nonjs.mbt

@@ -1470,12 +1470,13 @@ pub fn BigInt::from_octets(input : BytesView, signum? : Int = 1) -> BigInt {
     limbs[limbs_len - 1] = (limbs[limbs_len - 1] << 8) | input[i].to_uint()
   }
   let byte_per_limb = RADIX_BIT_LEN / 8
-  // tail
+  // tail: at RADIX_BIT_LEN == 32 a limb is exactly one big-endian 32-bit word,
+  // so the shift-accumulate loop collapses to a single u32be read. That
+  // constant is pinned by a test in bigint_nonjs_wbtest.mbt, so narrowing it
+  // fails there rather than silently mis-decoding here.
   for i in 0..<div {
-    for j in 0..<byte_per_limb {
-      let bytes_idx = len - byte_per_limb - i * byte_per_limb + j
-      limbs[i] = (limbs[i] << 8) | input[bytes_idx].to_uint()
-    }
+    guard! input[len - byte_per_limb - i * byte_per_limb:] is [u32be(word), ..]
+    limbs[i] = word
   }
   { limbs, sign: Positive, len: normalize_len(limbs, limbs_len), }
 }

bigint/bigint_nonjs_wbtest.mbt

@@ -182,3 +182,16 @@ test {
     content="{limbs : <FixedArray: [2, 2147483648, 0]>, sign : Negative, len : 2 }",
   ) // Int64.min_value - 2
 }
+
+///|
+/// `BigInt::from_octets` reads each tail limb with a single `u32be` bits
+/// pattern, which agrees with the shift-accumulate loop it replaced only when a
+/// limb is exactly four bytes wide. Pin the constant here so narrowing it fails
+/// this test instead of silently mis-decoding octets.
+///
+/// Deliberately `assert_eq` rather than `inspect`: a snapshot would be rewritten
+/// by `moon test --update`, which would silently retire this guard at exactly
+/// the moment it is supposed to fire.
+test "from_octets assumes 32-bit limbs" {
+  @test.assert_eq(RADIX_BIT_LEN, 32)
+}

bigint/from_octets_bench_test.mbt

@@ -0,0 +1,30 @@
+// Copyright 2026 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+///|
+fn from_octets_bench_data(len : Int) -> Bytes {
+  Bytes::makei(len, i => ((i * 37 + 11) & 0xff).to_byte())
+}
+
+///|
+test "bench BigInt::from_octets n=64" (it : @bench.T) {
+  let data = from_octets_bench_data(64)
+  it.bench(fn() { it.keep(@bigint.BigInt::from_octets(data)) })
+}
+
+///|
+test "bench BigInt::from_octets n=1024" (it : @bench.T) {
+  let data = from_octets_bench_data(1024)
+  it.bench(fn() { it.keep(@bigint.BigInt::from_octets(data)) })
+}