Draft builder API (ADR-009) and generalized validation validate_bytes (ADR-010) for v0.1.0

- ADR-009: fluent Rust builder producing serde_json::Value, covers
  AlkType kinds + standard JSON Schema; resolves OQ-003 (alkcall is the
  unblocking consumer)
- ADR-010: AlkTypeEngine::validate_bytes(&[u8]) as the single-call
  binary-buffer validation entry point; materialize Value from bytes,
  then validate; two methods on one struct, not a trait
- builder.md: full builder API spec (Schema, Definitions, Discriminator)
  with four usage examples (channels chunk header, call input schema,
  SFTP Packet union, OperationSpec error schemas)
- validation.md: new validate_bytes subsection + entry-point comparison
  table; AlkTypeEngine impl block updated; design decisions table updated
- overview.md: builder.md added to component pointers; 'Not a schema
  builder' scope boundary retired; ADR-009/010 added to decisions table;
  OQ-003 marked resolved; Consumers table adds alkcall as first consumer
- open-questions.md + questions/003: OQ-003 moved from deferred(scope)
  to resolved (ADR-009)
- README.md: builder doc + ADR-009/010 added; OQ-003 marked resolved;
  two new Key Design Principles (9, 10) for v0.1.0 additions

Doc-only change; 346 tests pass, clippy clean.

docs/architecture/questions/003-builder-api-for-schema-construction.md

@@ -4,23 +4,21 @@
   [../overview.md](../overview.md);
   `@alkdev/alknet: docs/research/alknet-typedef/findings.md` (the builder
   API was noted as the one detail not covered by the POCs)
-- **Status**: deferred(scope)
+- **Status**: resolved
 - **Door type**: Two-way (additive — a builder API can be added without
   changing the existing JSON-consumption path)
 - **Priority**: medium
-- **Impacts**: Blocks programmatic schema construction in Rust without a
-  JS toolchain. Any consumer that wants to build alktype schemas at
-  runtime from Rust code (rather than loading pre-authored JSON) must
-  construct the JSON manually or depend on TypeBox. Does NOT block any
-  current consumer — all v1 consumers (SFTP, metatensor, binary call
-  frames, TTY negotiation) use pre-authored schemas.
-- **Blocked on**: A concrete need for programmatic schema construction
-  in Rust. The current consumers (SFTP, metatensor, binary call frames,
-  TTY negotiation) all have schemas that can be hand-written or generated
-  from TypeBox.
-- **Resolution**: Not yet decidable. The builder API is important but
-  not needed for the initial consumers. The engine's JSON-consumption
-  path is the primary interface for v1. A builder API would be a fluent
-  Rust API that produces the same JSON Schema structure — it would sit
-  on top of the engine, not inside it.
-- **Cross-references**: ADR-001, [schema-layer.md](../schema-layer.md)
+- **Impacts**: (resolved) Was: blocks programmatic schema construction
+  in Rust without a JS toolchain.
+- **Blocked on**: (resolved) Was: a concrete need for programmatic
+  schema construction in Rust.
+- **Resolution**: Resolved by [ADR-009](../decisions/009-builder-api.md)
+  in v0.1.0. alkcall — the merged `alknet-call` + `alknet-channels`
+  extraction from `@alkdev/alknet` — is the concrete consumer that
+  unblocked the deferral: it builds binary-layout schemas (channels'
+  8-byte chunk header) and JSON payload schemas (`OperationSpec`
+  input/output/error schemas) at runtime from Rust code. The builder
+  produces `serde_json::Value`, covers both AlkType kinds and standard
+  JSON Schema, and is implemented in `src/builder.rs`. See
+  [../builder.md](../builder.md) for the spec.
+- **Cross-references**: ADR-001, ADR-009, [../builder.md](../builder.md)