adr-0046-docs-runtime-serving.md

@objectstack/cli	patch
@objectstack/metadata	patch

fix(ADR-0046): serve package docs at runtime, not just in the compiled artifact

Package docs (src/docs/*.md) compiled into a bundle were never reaching the runtime, so GET /meta/doc returned an empty list and the docs were invisible even though os build produced them.

Two gaps:

• os dev / os serve (config-load path) re-derives metadata from defineStack(...), which never carries the markdown docs — those are collected only at compile time. serve.ts now collects src/docs/*.md into the stack on the config-load path too (collection only — additive, never blocks boot), so docs serve in dev exactly as from a built artifact.
• The MetadataPlugin artifact loader (ARTIFACT_FIELD_TO_TYPE) omitted the docs → doc mapping, so the bundle's docs array was skipped when loading through that path. Added the mapping (with a regression test) for parity with the ObjectQL engine's metadataArrayKeys.