Add an end-to-end walkthrough of mutability, from CHL source to operators

docs/design.md

@@ -105,7 +105,7 @@ The APIs of library sources and sinks are **[Open]**.
 
 ### Mutability, time, and transactions
 
-Cambra's model of state is **temporal functional mutation**: a mutable variable *is* a total function from a **sequencing domain** (a time axis) to a value — its **history**. "Mutation" is the incremental revelation of that function as the domain advances; "reading" is lookup at a time. Conceptually, nothing is ever overwritten — there is no concept of a mutable cell in CCL: the compiler eliminates every mutable write into a pure, well-founded recursive definition whose cycles all pass through *causal accessors* (values at a position depend only on strictly earlier positions). This model is documented in [src/ccl/design/mutability.md](../src/ccl/design/mutability.md).
+Cambra's model of state is **temporal functional mutation**: a mutable variable *is* a total function from a **sequencing domain** (a time axis) to a value — its **history**. "Mutation" is the incremental revelation of that function as the domain advances; "reading" is lookup at a time. Conceptually, nothing is ever overwritten — there is no concept of a mutable cell in CCL: the compiler eliminates every mutable write into a pure, well-founded recursive definition whose cycles all pass through *causal accessors* (values at a position depend only on strictly earlier positions). This model is documented in [src/ccl/design/mutability.md](../src/ccl/design/mutability.md); [mutability-walkthrough.md](mutability-walkthrough.md) is a conceptual tour of the same ground, following two programs from CHL to operators.
 
 Mutability has two forms:
 - **Inductive** mutability is an exclusive mutable variable over a statically-determined time domain. The time domain is all of the writes to that variable.