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/overview.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-07-22
+last_updated: 2026-08-11
 ---
 
 # alktype — Overview
@@ -115,11 +115,20 @@ schema JSON determines the order of fields in the binary struct.
 
 | Consumer | Schema describes | Engine provides |
 |----------|-----------------|-----------------|
+| **alkcall** (v0.1.0 first consumer) | channels' `ChunkHeader { channel_id: u32 BE, length: u32 BE }` + call's `OperationSpec.input_schema` / `output_schema` / `error_schemas` | `validate_bytes` for the 8-byte chunk header; `validate_json` for call's JSON payloads; builder API for both |
 | russh-sftp | 29 packet structs + Packet union (byte discriminator) | Read/write SFTP frames from bytes |
 | metatensor | Model layout (ConvNet struct, tensor refs) | Offset map for mmap'd tensor access |
 | binary call frames | `call.requested` / `call.responded` / etc. structs | Read/write binary call frames |
 | TTY negotiation | `NegotiateRequest` / `NegotiateResponse` structs | Read/write TTY control frames |
-| channels wire | `ChunkHeader { channel_id, length }` | Already trivial (8 bytes, no schema needed) |
+| channels wire | `ChunkHeader { channel_id, length }` | 8-byte chunk header (now in scope for alkcall; was "trivial" pre-v0.1.0) |
+
+alkcall — the merged `alknet-call` (call protocol) + `alknet-channels`
+(channel multiplexing) extraction from `@alkdev/alknet` — is the
+consumer that bumped the builder API (OQ-003) and generalized
+validation (ADR-010) into v0.1.0. It uses alktype for two distinct
+schema roles (binary layout + JSON payloads) from one library. See
+[builder.md](builder.md) and [validation.md](validation.md)
+§"validate_bytes".
 
 The russh-sftp case is the most instructive and the highest-value POC
 target. The `Packet` enum's `TryFrom<&mut Bytes>` impl is a hand-written
@@ -142,10 +151,13 @@ These boundaries are decided in [ADR-001](decisions/001-alktype-purpose-scope-js
   later.
 - **Not a code generator.** typebox-rs's `codegen/` module is a separate
   concern. The alktype engine consumes schemas; it does not generate them.
-- **Not a schema builder.** The alktype engine does not provide a fluent
-  API for constructing schemas. Schemas are plain JSON — authored in
-  TypeBox, generated by ujsx components, or hand-written. A builder API
-  is deferred (OQ-003).
+- **Schema builder is in scope as of v0.1.0.** A fluent Rust API for
+  constructing schemas at runtime, producing `serde_json::Value`, is
+  shipped in v0.1.0 ([ADR-009](decisions/009-builder-api.md), resolves
+  OQ-003). The builder covers AlkType kinds and standard JSON Schema;
+  see [builder.md](builder.md). Schemas may still be authored in
+  TypeBox, generated by ujsx components, or hand-written — the builder
+  is an additional construction path, not a replacement.
 - **Not a serialization framework.** The alktype engine is not a
   general-purpose serde replacement. It operates on raw byte buffers at
   computed offsets — no intermediate `Value` tree, no reflection, no
@@ -166,6 +178,11 @@ These boundaries are decided in [ADR-001](decisions/001-alktype-purpose-scope-js
 - **[validation.md](validation.md)** — custom keyword validators for all
   19 `AlkType:*` kinds, `AlkTypeError`, load-time vs access-time
   validation, `AlkTypeEngine` as the compiled form of a schema.
+  `validate_json` for JSON values; `validate_bytes` for binary buffers
+  (ADR-010).
+- **[builder.md](builder.md)** — fluent Rust API for constructing
+  alktype JSON Schemas at runtime, producing `serde_json::Value`.
+  Covers AlkType kinds and standard JSON Schema (ADR-009).
 
 ## Design Decisions
 
@@ -179,6 +196,8 @@ These boundaries are decided in [ADR-001](decisions/001-alktype-purpose-scope-js
 | Non-final inline variable fields | [ADR-006](decisions/006-reject-non-final-inline-length-prefixed-in-aligned-mode.md) | Rejected in aligned mode (would clobber subsequent fields) |
 | Packed-mode read factory | [ADR-007](decisions/007-packed-mode-read-factory.md) | `engine.sequential_reader()` returns an owned fresh reader |
 | TUnion in aligned mode | [ADR-008](decisions/008-reject-tunion-in-aligned-mode.md) | Rejected for v1 (broken semantics; no current consumer needs it) |
+| Builder API | [ADR-009](decisions/009-builder-api.md) | Fluent Rust API producing `serde_json::Value`; covers AlkType kinds + standard JSON Schema; resolves OQ-003 |
+| Generalized validation — `validate_bytes` | [ADR-010](decisions/010-generalized-validation-validate-bytes.md) | Single-call binary-buffer validation on `AlkTypeEngine`; materialize `Value` from bytes, then validate |
 
 ## Open Questions
 
@@ -186,7 +205,9 @@ See [open-questions.md](open-questions.md) for full details.
 
 - **OQ-001** (deferred(scope)): Arrays of variable-length-element structs.
 - **OQ-002** (deferred(scope)): `no_std` + `alloc` support.
-- **OQ-003** (deferred(scope)): Builder API for schema construction.
+- **OQ-003** (resolved by [ADR-009](decisions/009-builder-api.md)):
+  Builder API for schema construction. Shipped in v0.1.0; see
+  [builder.md](builder.md).
 
 ## References