docs(architecture): add alknet-typedef crate specs, ADRs 095-098, and OQs 069-071

Add the alknet-typedef architecture specification — the binary struct
engine that takes JSON Schema with TypeDef:* custom keywords and produces
offset maps, read/write functions, and validation.

Specs (docs/architecture/crates/typedef/):
- overview.md: purpose, 'schema is the format' principle, consumers, scope
- schema-layer.md: 17 TypeDef:* kinds, jsonschema integration, annotations
- layout-engine.md: two layout modes, three variable-length strategies
- data-access.md: read/write, TUnion dispatch, field paths, zero-copy
- validation.md: custom keyword validators, TypedefError, TypedefEngine

ADRs:
- 095: Purpose, scope, and the jsonschema engine
- 096: Two layout modes — packed sequential vs aligned static
- 097: Schema annotations — endianness, alignment, encoding, TUnion
- 098: Error handling and validation strategy

OQs (deferred(scope)):
- 069: Arrays of variable-length-element structs
- 070: no_std + alloc support
- 071: Builder API for schema construction

Index updates: README doc table + ADR table, open-questions.md theme
table + Deferred/Blocked section, overview.md crate graph.

Grounded in the alknet-typedef POC (26 tests passing) and the
call-channels-unification research. Reviewed by architecture-reviewer;
all critical issues, warnings, and suggestions addressed.

docs/architecture/questions/071-builder-api-for-schema-construction.md

@@ -0,0 +1,24 @@
+# OQ-071: Builder API for schema construction
+
+- **Origin**: [crates/typedef/schema-layer.md](crates/typedef/schema-layer.md),
+  [crates/typedef/overview.md](crates/typedef/overview.md);
+  `docs/research/alknet-typedef/findings.md` (the builder API was noted
+  as the one detail not covered by the POCs)
+- **Status**: deferred(scope)
+- **Door type**: Two-way (additive — a builder API can be added without
+  changing the existing JSON-consumption path)
+- **Priority**: medium
+- **Impacts**: No current consumer. Schemas are authored in TypeBox (JS)
+  or hand-written JSON for v1. A Rust builder API would enable
+  programmatic schema construction in Rust without depending on a JS
+  toolchain, but no current consumer needs this.
+- **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-095, [schema-layer.md](crates/typedef/schema-layer.md)