feat(doc): Update engine architecture documentation (#970)

Author: aapoalas

ARCHITECTURE.md

@@ -4,59 +4,27 @@ The architecture of Nova engine is built around data-oriented design. This means
 that most "data" like things are found in the heap in a vector with like-minded
 individuals.
 
-## Concurrency
-
-Currently the heap is not thread-safe at all but the plan is to make it
-thread-safe enough for concurrent marking to become possible in the future. To
-this end, here's how I (@aapoalas) envision the engine to look like in the
-future:
-
-```rs
-struct Agent<'agent, 'generation>(Box<AgentInner<'agent>>, AgentGuard<'generation>);
-```
-
-The `Agent` struct here is just a RAII wrapper around the inner Heap-allocated
-Agent data. The first important thing is the `'agent` lifetime: This is a brand.
-It is valid for as long as the Nova engine instance lives, and its only purpose
-is to make sure that (type-wise) uses cannot mistakenly or otherwise mix and
-match Values from different engine instances.
-
-The second lifetime, `'generation`, is the garbage collection generation. Here
-what I want to achieve is a separation between "gc" and "nogc" scopes. But
-before we dive into that, here's what I imagine a `Value` looking like:
-
-```rs
-enum Value<'generation> {
-    Undefined,
-    String(StringIndex<'generation>>),
-    SmallString(SmallString),
-    // ...
-}
-```
-
-The `Value` enum carries the `'generation` lifetime: As long as we can guarantee
-that no garbage collection happens, we can safely keep `Value<'gen>` on the
-stack or even temporarily on the heap.
-
-If we call a method that may trigger GC, then all `Value<'gen>` items are
-invalidated. If we want to keep values alive through eg. JavaScript function
-calls, we must use:
-
-```rs
-struct ShadowStackValue<'agent>(u32, PhantomPinned);
-```
-
-This just moves the `Value` onto an Agent-controlled "shadow stack" that the
-`u32` points into. Due to the `PhantomPinned` the shadow stack is mostly just
-push-pop as any stack should be, and thus relatively quick. But it is also on
-the heap and thus garbage collection can update any references on the shadow
-stack.
-
-Note that this is essentially equivalent to:
-
-```rs
-struct GlobalValue<'agent>(u32);
-```
-
-but "global values" are not push-pop, likely will have generational indexes,
-possibly will have reference counting and so on and so forth.
+## ECMAScript implementation
+
+Nova code aims to conform fairly strictly to the
+[ECMAScript specification](https://tc39.es/ecma262/) in terms of both code
+layout and structure.
+
+For details on the ECMAScript implementation, see the
+[ecmascript/README.md](./nova_vm/src/ecmascript/README.md).
+
+## Engine implementation
+
+Nova's VM is a stack-based bytecode interpreter.
+
+For details on the engine, see the
+[engine/README.md](./nova_vm/src/engine/README.md).
+
+## Heap implementation
+
+Nova's heap is made up of a mix of normal Rust `Vec`s and custom `SoAVec`
+structs that implement a "Struct of Arrays" data structure with an API
+equivalent to normal `Vec`s, all referenced by-index.
+
+For details on the heap architecture, see the
+[heap/README.md](./nova_vm/src/heap/README.md).