[style](trx-ftx): use iterators in LDPC single-index loops

clippy needless_range_loop (rust 1.97) flagged the loops in ldpc_check
and ldpc_decode that use a range only to index one array. Replace them
with iterator/enumerate forms. The belief-propagation loops that index
several arrays by the same variable are left as-is (not flagged).

Behaviour is unchanged; the transformations are index-for-index
equivalent. Verified the lib compiles and is clippy-clean; the crate's
LDPC tests run in the CI test job (they need a dev-dependency not
available in the local offline sandbox).

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

src/decoders/trx-ftx/src/common/ldpc.rs

@@ -42,8 +42,8 @@ pub(crate) fn ldpc_check(codeword: &[u8; FTX_LDPC_N]) -> i32 {
     for m in 0..FTX_LDPC_M {
         let mut x: u8 = 0;
         let num_rows = FTX_LDPC_NUM_ROWS[m] as usize;
-        for i in 0..num_rows {
-            x ^= codeword[FTX_LDPC_NM[m][i] as usize - 1];
+        for &nm in FTX_LDPC_NM[m].iter().take(num_rows) {
+            x ^= codeword[nm as usize - 1];
         }
         if x != 0 {
             errors += 1;
@@ -81,11 +81,11 @@ pub fn ldpc_decode(
         for j in 0..FTX_LDPC_M {
             let num_rows = FTX_LDPC_NUM_ROWS[j] as usize;
             let m_row = j * FTX_LDPC_N;
-            for ii1 in 0..num_rows {
-                let i1 = FTX_LDPC_NM[j][ii1] as usize - 1;
+            for &nm1 in FTX_LDPC_NM[j].iter().take(num_rows) {
+                let i1 = nm1 as usize - 1;
                 let mut a = 1.0f32;
-                for ii2 in 0..num_rows {
-                    let i2 = FTX_LDPC_NM[j][ii2] as usize - 1;
+                for &nm2 in FTX_LDPC_NM[j].iter().take(num_rows) {
+                    let i2 = nm2 as usize - 1;
                     if i2 != i1 {
                         a *= fast_tanh(-m_matrix[m_row + i2] / 2.0f32);
                     }
@@ -97,8 +97,8 @@ pub fn ldpc_decode(
         // Hard decisions
         for i in 0..FTX_LDPC_N {
             let mut l = codeword[i];
-            for j in 0..3 {
-                l += e_matrix[(FTX_LDPC_MN[i][j] as usize - 1) * FTX_LDPC_N + i];
+            for &mn in FTX_LDPC_MN[i].iter().take(3) {
+                l += e_matrix[(mn as usize - 1) * FTX_LDPC_N + i];
             }
             plain[i] = if l > 0.0 { 1 } else { 0 };
         }
@@ -113,12 +113,12 @@ pub fn ldpc_decode(
 
         // Update m[][] from e[][]
         for i in 0..FTX_LDPC_N {
-            for ji1 in 0..3 {
-                let j1 = FTX_LDPC_MN[i][ji1] as usize - 1;
+            for (ji1, &mn1) in FTX_LDPC_MN[i].iter().enumerate().take(3) {
+                let j1 = mn1 as usize - 1;
                 let mut l = codeword[i];
-                for ji2 in 0..3 {
+                for (ji2, &mn2) in FTX_LDPC_MN[i].iter().enumerate().take(3) {
                     if ji1 != ji2 {
-                        let j2 = FTX_LDPC_MN[i][ji2] as usize - 1;
+                        let j2 = mn2 as usize - 1;
                         l += e_matrix[j2 * FTX_LDPC_N + i];
                     }
                 }