Add quadrature encoder starter course

lessons/01-reading-encoder-signals/02-counting-direction-and-steps/counting-direction-and-steps.md

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+---
+title: Counting Direction and Steps
+summary: Decode edges from the A and B channels to recover step count and direction.
+chapter: Reading Encoder Signals
+order: 2
+tags:
+  - decoding
+  - interrupts
+  - firmware
+estimated_minutes: 10
+---
+
+# Counting Direction and Steps
+
+Once you can read the A and B channels, the next job is decoding.
+
+The most common approach is:
+
+- trigger on an edge
+- read both channels
+- use a lookup table or state machine to update the count
+
+If your decoder sees illegal state jumps, you are probably dropping edges or reading a noisy signal.
+
+## Practical notes
+
+- Start with low shaft speed.
+- Add debounce or filtering only if the hardware needs it.
+- Test forward and reverse explicitly.
+
+## Checkpoint
+
+Write firmware that increments a counter when the shaft turns forward and decrements when it turns backward.