Deepen the storage provider extension contract

[ian-pascoe]

Problem

Provider extension and lifecycle safety still rely on weak seams:

• coordinator/CLI fallback distinguishes an unregistered provider by matching error message text ("is not registered"); a registered factory throwing the same text can be silently replaced by a built-in;
• lifecycle restore/migration needs target namespace and logical schema identity, but WritableAuthority does not require both metadata fields, forcing casts/fallbacks for alternate providers.

Acceptance criteria

• Factory lookup exposes a typed registry-miss result/error and never inspects human error messages.
• CLI and coordinator share one fallback helper.
• WritableAuthority requires target namespace and schema version (or a required typed lifecycle identity capability) for every registered provider.
• Alternate-provider contract tests prove factory failures propagate and lifecycle identity mismatches fail closed.

Evidence

Review anchors: packages/core/src/storage/coordinator.ts around provider fallback (~619) and packages/core/src/storage/backup.ts around target identity (~176).