docs: godoc and package-doc pass

[devarismeroxa]

Summary

Docs-only pass on the processor SDK. No behavior, signature, or logic changes.

New doc.go files (2):

• doc.go — root sdk package. Covers purpose, how to implement a Processor (embed UnimplementedProcessor), the method lifecycle and ordering, the ProcessedRecord return/error-propagation contract, the standalone (WASM) vs built-in hosting model, the schema encode/decode middleware, and neighboring packages.
• schema/doc.go — schema package. Covers Get/Create, how SchemaService differs between standalone (host registry) and built-in/test (in-memory) hosting, and caching.

Godoc added/improved on exported symbols (~9):

• ErrFilterRecord — sentinel semantics (filter vs. fail) for NewProcessorFunc functions.
• ProcessorFunc.Process — documents the truncate-on-first-error behavior (returned slice is shorter than input; remaining records are left for reprocessing), which is non-obvious from the signature.
• schema: Get, Create, TypeAvro, SchemaService, ErrSubjectNotFound, ErrVersionNotFound, ErrInvalidSchema, InMemoryService, NewInMemoryService.

Highest-value contracts documented

• Process lifecycle & ordering: Specification → Configure → Open → Process (repeated) → Teardown, called sequentially by the standalone command loop; idempotency requirement on Process.
• Error propagation: how each ProcessedRecord type (SingleRecord / MultiRecord / FilterRecord / ErrorRecord) routes a record — continue / split / ack-and-drop / nack-to-DLQ — and that dropping an unprocessable record instead of returning ErrorRecord violates at-least-once.
• WASM constraints: standalone processors compile to GOOS=wasip1 GOARCH=wasm and call Run as their entry point; built-in processors are invoked directly and skip the WebAssembly boundary; author code is identical in both modes.

Deliberately not documented

Skipped one-liner getters like SchemaPayloadEnabledParameterName / SchemaKeyEnabledParameterName and the wasm/pprocutils internal plumbing (their package docs already say "DO NOT use directly") — adding comments there would restate the name without adding information, against the "say something the signature doesn't" bar.

Verification

• go build ./... — clean
• go test ./... -count=1 — pass
• GOOS=wasip1 GOARCH=wasm go build ./... — clean
• make lint (golangci-lint v2.12.2) — 0 issues (baseline was also 0)

Risk tier: Tier 3 (docs/comments only). No data-path or contract change.