Initial commit: telfax - Rust Fax Server

- Modem driver (serialport) with AT command interface
- Class 1 fax protocol (T.30 session, HDLC framing)
- T.4/T.6 image codec (fax crate integration)
- Document conversion (image to fax, TIFF-F output)
- REST API (axum) for fax job management
- SQLite/InMemory job queue
- CLI interface (detect/serve/send/test)
- Integration tests passing

src/fax/t4.rs

@@ -0,0 +1,106 @@
+use crate::error::{FaxError, Result};
+
+pub struct T4Codec;
+
+#[derive(Debug, Clone)]
+pub struct T4Page {
+    pub data: Vec<u8>,
+    pub width_pels: u32,
+    pub rows: u32,
+    pub encoding: T4Encoding,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum T4Encoding {
+    Group3MH,
+    Group3MR,
+    Group4MMR,
+}
+
+impl T4Codec {
+    /// Decode T.4/T.6 data into raw 1-bit pixel data (MSB-first, row-major).
+    /// `transitions_to_pixels` converts run-length transitions to pixel data.
+    fn transitions_to_pixels(
+        transitions: &[u16],
+        width_pels: u32,
+        row_buf: &mut [u8],
+    ) {
+        row_buf.fill(0);
+        let mut is_black = false;
+        let mut prev = 0u32;
+
+        for &t in transitions {
+            let t = t as u32;
+            if t > width_pels {
+                break;
+            }
+            if is_black {
+                for p in prev..t.min(width_pels) {
+                    let byte_idx = (p / 8) as usize;
+                    let bit_idx = 7 - (p % 8);
+                    if byte_idx < row_buf.len() {
+                        row_buf[byte_idx] |= 1 << bit_idx;
+                    }
+                }
+            }
+            prev = t;
+            is_black = !is_black;
+        }
+        // If ends on black, fill to end
+        if is_black {
+            for p in prev..width_pels {
+                let byte_idx = (p / 8) as usize;
+                let bit_idx = 7 - (p % 8);
+                if byte_idx < row_buf.len() {
+                    row_buf[byte_idx] |= 1 << bit_idx;
+                }
+            }
+        }
+    }
+
+    pub fn decode(data: &[u8], width_pels: u32, rows: u32, encoding: T4Encoding) -> Result<Vec<u8>> {
+        let bytes_per_row = ((width_pels + 7) / 8) as usize;
+        let mut pixels = vec![0u8; bytes_per_row * rows as usize];
+        let data: Vec<u8> = data.to_vec();
+
+        match encoding {
+            T4Encoding::Group3MH | T4Encoding::Group3MR => {
+                let mut row_idx = 0u32;
+                let result = fax::decoder::decode_g3(data.into_iter(), |transitions: &[u16]| {
+                    if (row_idx as usize) < pixels.len() / bytes_per_row {
+                        let start = row_idx as usize * bytes_per_row;
+                        let end = (start + bytes_per_row).min(pixels.len());
+                        let row_buf = &mut pixels[start..end];
+                        Self::transitions_to_pixels(transitions, width_pels, row_buf);
+                    }
+                    row_idx += 1;
+                });
+                match result {
+                    Some(()) => Ok(pixels),
+                    None => Err(FaxError::t4("Group 3 decode returned None")),
+                }
+            }
+            T4Encoding::Group4MMR => {
+                let mut row_idx = 0u32;
+                let result = fax::decoder::decode_g4(
+                    data.into_iter(),
+                    width_pels as u16,
+                    None,
+                    |transitions: &[u16]| {
+                        if (row_idx as usize) < pixels.len() / bytes_per_row {
+                            let start = row_idx as usize * bytes_per_row;
+                            let end = (start + bytes_per_row).min(pixels.len());
+                            let row_buf = &mut pixels[start..end];
+                            Self::transitions_to_pixels(transitions, width_pels, row_buf);
+                        }
+                        row_idx += 1;
+                    },
+                );
+                match result {
+                    Some(()) => Ok(pixels),
+                    None => Err(FaxError::t4("Group 4 decode returned None")),
+                }
+            }
+        }
+    }
+}