docs: codegen output and packaging-layer design specs

[OmarAlJarrah]

Summary

Adds design specifications for the output and packaging layer of a future code generator that
will emit per-API SDKs on top of the hand-written sdk-core toolkit. These are design docs only
under docs/codegen/ — no generator, KotlinPoet, or schema/validation code is introduced, and
nothing here ships in sdk-core. Every code-shaped snippet is labelled as illustrative target
generator output, not compiled in this repo.

Each spec is grounded in the real sdk-core API (Serde, Tristate, RequestBody, Paginator,
the HttpClient / AsyncHttpClient SPIs, HttpPipeline / HttpPipelineBuilder, the Io seam, and
the context chain) so the eventual generator targets the current surface.

Docs added

• strict-structured-output-schema.md — strict JSON-schema encoding rules for structured
  outputs: all-required, additionalProperties:false, optional-as-nullable-union, deterministic
  $defs names, and how Tristate collapses to value | null on the wire. Derivation and the
  hand-rolled subset validator live in a Jackson adapter, never sdk-core.
• fail-soft-validator-skeleton.md — design of a reusable fail-soft recursive validator
  (one-shot recursion guard, path-prefixed error list, early-return verify helper, deterministic
  definition names). Deferred: design only, no runtime type added today.
• generated-sdk-provenance.md — a provenance file (generator version + normalized
  input-contract hash, plus supporting metadata), its JSON format, and its classpath + source-root
  locations. Stamped into generated output only, never the toolkit.
• spring-boot-starter.md — per-API Spring Boot starter shape: a @ConfigurationProperties
  class, a fun interface customizer, and an @AutoConfiguration with @Bean @ConditionalOnMissingBean
  that assembles {IoProvider + transport + HttpPipeline}. Spring deps confined to the generated
  starter module.
• README.md — index linking the four specs and listing the sdk-core anchors they build on.

Notes

Docs only — no source, build, or public-API changes.

Closes #66
Closes #67
Closes #68
Closes #69