ADR-0048 follow-up: package-scoped metadata overlays (env-wide overlay can't disambiguate same-name collisions) #1824

Context

ADR-0048 (package-id as route/identity key) landed across #1810, #1813, #1816, #1819, #1822 (+ objectui #1708, #1711). Single-item reads (?package=), the Studio editor, and the runtime render surface (dashboard/report/page/doc via prefer-local) now resolve same-name collisions to the correct package — for pure-artifact items (the marketplace default).

The remaining gap

A package-less, env-wide customization overlay (a sys_metadata row with package_id NULL) for a name that collides across two installed packages is inherently ambiguous and cannot be disambiguated today.

Root cause is schema-level: sys_metadata has a UNIQUE index on (type, name, organization_id) (idx_sys_metadata_overlay_active / …_draft), not including package_id. So:

• At most ONE active overlay row can exist per (type, name, org).
• restoreMetadataFromDb rehydrates that row under the bare registry key (no package), where it takes ADR-0005 overlay precedence over BOTH packages' composite artifacts.
• A getMetaItem(type, name, packageId) whose overlay query misses the scoped package_id falls back to the package-less row and serves the same overlay for every package.

Concretely: showcase ships page/home, studio ships page/home; an admin customizes one → the env overlay shadows both, and ?package= can't tell them apart.

Proposed direction (needs an ADR / design decision)

Make overlays package-aware:

• Add package_id to the overlay uniqueness key: UNIQUE(type, name, organization_id, package_id) (migration + backfill).
• saveMetaItem binds the overlay to the edited item's owning package (the Studio editor already threads ?package= since docs(references): regenerate flow reference for the map node type #1711 — the write path needs to persist it).
• restoreMetadataFromDb registers package-bound overlays under the composite key, not bare.
• protocol.getMetaItem / getMetaItemLayered overlay queries already prefer package_id-scoped rows (ADR-0048); the package-less fallback should only apply to genuinely package-less (legacy/global) overlays.

Open questions

• Migration story for existing package-less overlay rows (treat as global? attribute to first-match package? leave as-is with a lint?).
• Should a package-less overlay still be allowed (an intentional "apply to all same-named items" escape hatch), or is every overlay now package-bound?
• Interaction with org-scoped vs env-wide precedence (ADR-0005).

Acceptance

• Two installed packages each shipping page/home, each with its own overlay, resolve independently via ?package= and in the Studio layered/editor view.
• ADR-0005 overlay precedence preserved for the single-package case.
• Browser-verified with a real collision (as fix(metadata): keep each colliding item's own _packageId provenance (ADR-0048) #1822 was).

Out of scope: pure-artifact collisions (already fixed in #1822).