Add rewatch overview document

This document presents rewatch's core algorithms as typed functions,
abstracting implementation details to expose fundamental computational
patterns. It includes:

- Type vocabulary defining the build system's state space
- Diagrams illustrating one-shot build and watch mode flows
- Algorithms for package/module discovery, AST generation,
  dependency analysis, and topological compilation
- Discussion of patterns: graph traversal, parallel map,
  dirty propagation, ABI fingerprinting, topological scheduling
- Section on mapping to incremental/reactive computation primitives

The goal is to identify algorithmic building blocks that could be
expressed using incremental and reactive computation primitives.

Author: cristianoc