Encoder: Implement a Run-Length and Custom Encoding Scheme for Strings

Question

## Problem

Implement a two-pass encoder/decoder. Given a string, first apply run-length encoding (consecutive repeated chars become `char+count`), then apply a substitution cipher where each encoded character is shifted by its 1-based position in the output string (mod 26, letters only, leave digits unchanged).

```python
def encode(s: str) -> str: ...
def decode(s: str) -> str: ...
```

```
Encode steps for "aaabbc":
  Step 1 (RLE):       "a3b2c1"
  Step 2 (shift):     position 1='a' shift 1->'b', digit '3' unchanged,
                      position 3='b' shift 3->'e', digit '2' unchanged,
                      position 5='c' shift 5->'h', digit '1' unchanged
  Final:              "b3e2h1"

Decode("b3e2h1") -> "aaabbc"
```

## Follow-ups

1. Your decoder must invert both passes in reverse order. Walk through the decode algorithm step by step.
2. What inputs break your RLE step? (Hint: digits in the original string.)
3. How would you make the encoder streamable — processing characters without loading the full string into memory?
4. If you needed to transmit the encoded string over a network with a 100-byte MTU, how would you chunk and reassemble it safely?

Encoder: Implement a Run-Length and Custom Encoding Scheme for Strings

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