notion-md: remove dead unsupported-block/object-store storage subsystem + FrontmatterV1 legacy (coordinate with #775)

Two redundancy/legacy clusters surfaced by analysis, both intersecting PR #775 (its storage / durable-artifact clean-break) — so coordinate with #775 before removing.

Dead storage subsystem (refuse-lossy orphan)

Since refuse-lossy (decisions 0016/0017), pages with unknown blocks classify lossy and are refused by assertRemoteMarkdownComplete before any storage write — so storage is always emptyStorage() and decideStorage can only return keep_self_contained. Dead on every live path:

• the object_store storage variant, writeStorageObject, NmdStorageObjectV2, objectPath / makeNmdObjectRef
• the requires_object_store branch in storage-policy.ts / sync.ts decideStorage
• Needs-a-decision: persisted object_store on-disk read-compat (keep for old .nmd files, or drop under #775 clean-break?)

FrontmatterV1 legacy

NmdFrontmatterV1, decodeNmdFrontmatterV1/V1Sync, classifyNmdFrontmatterPayload — zero non-test importers; runtime is V2-exclusive. (The redundancy analysis notes #775 clean-break authorizes this.) NOTE: NmdSyncStateV1 is the live current sidecar — do NOT confuse/remove it.

Also

Unused prebuilt decoder exports (decodeNmdFrontmatterV2/V2Sync, decodeNmdSyncStateV1); the notion-effect-client public service exports with no internal consumer (NotionComments/Users/Search/Files/…) are a separate semver call, not in scope here.

Deferred from the #786 editor/consolidation work; coordinate with #775 (cl1-basin) on the storage model before deleting.

Coordinate with #775 first, then read storage-policy.ts and sync.ts and trace the listed object-store and FrontmatterV1 symbols, including their test-only importers. Done means removing only the dead storage and FrontmatterV1 paths under the agreed compatibility policy, while preserving the live NmdSyncStateV1 sidecar and keeping unrelated public exports out of scope.