V1.0: Class 1 fax — real-world 4-page send to external number confirmed

Core features:
- Class 1 T.30 protocol: full send/receive implementation
- HDLC: DLE-stuffing, FCS strip, USR5637 bit-reversal handling
- T.4 MH encoder/decoder (1728px A4 standard)
- Document pipeline: PDF (Ghostscript), PNG, TIFF input
- Width clamping: US Letter 1734px → 1728px fax standard
- Cover page: CJK rasterization (TW/CN/JP/EN), TIFF + HTML output
- OCR verification: Tesseract 5 with eng+chi_tra, CJK space-tolerant
- API server (axum): health, send, jobs, cover, retry, cancel
- Background worker: auto-poll queue, speed fallback, retry policy
- Modem detection, pool management

Real-world test results (2026-07-23):
- V90 → 25153038: 4 pages, V.17 12000 bps, 2:33 ✅
- USR5637 → 25153038: 4 pages, V.17 12000 bps, 2:26 ✅
- Both faxes confirmed received on remote machine

Tested: loopback (100% pixel match), multi-page, all input formats,
cover pages, OCR verify, API endpoints, worker processing.
13 unit tests pass, 0 new clippy warnings.

src/worker/speed_fallback.rs

@@ -0,0 +1,59 @@
+use crate::fax::negotiate::DataRate;
+
+#[derive(Debug, Clone)]
+pub struct SpeedFallback {
+    rates: Vec<DataRate>,
+    current_index: usize,
+}
+
+impl Default for SpeedFallback {
+    fn default() -> Self {
+        Self {
+            rates: vec![
+                DataRate::V17_14400,
+                DataRate::V17_12000,
+                DataRate::V29_9600,
+                DataRate::V29_7200,
+                DataRate::V27ter4800,
+            ],
+            current_index: 0,
+        }
+    }
+}
+
+impl SpeedFallback {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn current_rate(&self) -> DataRate {
+        self.rates[self.current_index]
+    }
+
+    pub fn lower_speed(&mut self) -> Option<DataRate> {
+        if self.current_index + 1 < self.rates.len() {
+            self.current_index += 1;
+            Some(self.rates[self.current_index])
+        } else {
+            None
+        }
+    }
+
+    pub fn reset(&mut self) {
+        self.current_index = 0;
+    }
+
+    pub fn is_min_speed(&self) -> bool {
+        self.current_index == self.rates.len() - 1
+    }
+
+    pub fn set_initial_rate(&mut self, rate: DataRate) {
+        for (i, r) in self.rates.iter().enumerate() {
+            if *r == rate {
+                self.current_index = i;
+                return;
+            }
+        }
+        self.current_index = 0;
+    }
+}