Split single pydantic.md into pydantic/ with:
- README.md — core idea, when to use what
- settings.md — BaseSettings, nested config, splitting by concern
- validation.md — Annotated types as reusable type library (from prefect/types/)
- serialization.md — model_dump, JSON round-trips, TypeAdapter

Preps for plyr.fm config.py decomposition (phase 3).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

split io.md into io/ with four files based on reading the official
std library docs and devlog:
- concurrency.md: async/concurrent, Future, Group, Select, Queue
- synchronization.md: Mutex, Condition, CancelProtection, cancellation
- patterns.md: InitOptions, backend selection, debug_io, lifecycle

update relay-integration.md with Io boundary table showing which
pipeline stages use io.concurrent vs std.Thread.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

readLinkAbsolute returns usize, makePath → createDirPath,
ArrayList .{} → .empty, Server.deinit, Stream.Reader.interface,
File.readStreaming scatter/gather API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>