[v0.2.0] One-page architecture doc

[neverDefined]

Problem

A drive-by contributor opening anvil.go (760+ lines, all public API + lifecycle + builder) has no map of how the pieces fit. CLAUDE.md captures conventions but doesn't explain what's where.

Proposal

Add docs/architecture.md — one page, no fluff. Sections:

1. One-paragraph mental model — anvil is a subprocess; we own its lifecycle and expose a Go-typed RPC client.
2. Lifecycle diagram (ASCII): Build() → Start() → [ready] → Stop()/Close(). Where the cancel context lives. Where sync.Once kicks in.
3. File map — what each .go file is responsible for (mirror the table in CLAUDE.md but with one-sentence summaries).
4. RPC method anatomy — annotated copy of the canonical wrapper showing rpcCalls.Add, CallContext, error wrap, log.
5. Test strategy — shared-anvil pattern, when to use setupTestAnvil instead, the unit-test httptest pattern.
6. Where to add new things — RPC wrapper → anvil.go + tests; new builder option → builder section + validate(); new helper → helpers.go.

Acceptance criteria

• One page, scannable in 60 seconds.
• Linked from CONTRIBUTING.md and CLAUDE.md.
• No diagrams that need rendering — ASCII or none.