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/negotiate.rs

@@ -0,0 +1,91 @@
+#[derive(Debug, Clone)]
+pub struct FaxCapabilities {
+    pub resolution: Resolution,
+    pub page_width: PageWidth,
+    pub page_length: PageLength,
+    pub data_rate: DataRate,
+    pub ecm: bool,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Resolution {
+    Standard,  // 98 lpi, 204 dpi
+    Fine,      // 196 lpi, 204 dpi
+    SuperFine, // 392 lpi, 204 dpi
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PageWidth {
+    A4,  // 1728 pels
+    A3,  // 2432 pels
+    B4,  // 2048 pels
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PageLength {
+    A4,      // 297mm
+    Unlimited,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DataRate {
+    V27ter2400,
+    V27ter4800,
+    V29_7200,
+    V29_9600,
+    V17_7200,
+    V17_9600,
+    V17_12000,
+    V17_14400,
+}
+
+impl DataRate {
+    pub fn to_modulation_value(&self) -> u32 {
+        match self {
+            DataRate::V27ter2400 => 24,
+            DataRate::V27ter4800 => 48,
+            DataRate::V29_7200 => 72,
+            DataRate::V29_9600 => 96,
+            DataRate::V17_7200 => 73,
+            DataRate::V17_9600 => 97,
+            DataRate::V17_12000 => 121,
+            DataRate::V17_14400 => 145,
+        }
+    }
+
+    pub fn from_modulation_value(val: u32) -> Option<Self> {
+        match val {
+            24 => Some(DataRate::V27ter2400),
+            48 => Some(DataRate::V27ter4800),
+            72 => Some(DataRate::V29_7200),
+            96 => Some(DataRate::V29_9600),
+            73 => Some(DataRate::V17_7200),
+            97 => Some(DataRate::V17_9600),
+            121 => Some(DataRate::V17_12000),
+            145 => Some(DataRate::V17_14400),
+            _ => None,
+        }
+    }
+
+    pub fn max_for_modem(supported_rates: &[u32]) -> Option<Self> {
+        let priority = [145, 121, 97, 73, 96, 72, 48, 24];
+        for rate in &priority {
+            if supported_rates.contains(rate) {
+                return Self::from_modulation_value(*rate);
+            }
+        }
+        None
+    }
+}
+
+impl Default for FaxCapabilities {
+    fn default() -> Self {
+        Self {
+            resolution: Resolution::Fine,
+            page_width: PageWidth::A4,
+            page_length: PageLength::A4,
+            data_rate: DataRate::V17_14400,
+            ecm: true,
+        }
+    }
+}