Docs: surface adopter constraints (registry.extend for new top-level types; refs index only object.*; dbImport/dialect required for DB-less projects)

[dmealing]

Three constraints surfaced while building a real consumer project (a CLI system monitor — all value objects, no database). Each was resolvable, but none was obvious from the docs.

1. metadata.root childRules are closed

Adding new top-level node types (e.g. custom adapter.* / probe.* subtypes via a provider) requires registry.extend; without it the model fails to load with ERR_CHILD_NOT_ALLOWED. Worth a short "defining custom top-level types" doc section.

2. Declarative reference resolution only indexes object.* nodes

Cross-references from a template/adapter to a non-object.* node aren't resolved by the built-in resolver and must be hand-walked in a provider validate hook. Adopters modeling references to non-object nodes should know this up front.

3. MetaobjectsGenConfig requires dbImport + dialect even when no DB code is generated

A project that declares only value objects (no write-through entities) generates zero DB / query / route code — yet omitting dbImport/dialect is a tsc type error, so they end up as dead-but-mandatory config.

Ask: make dbImport/dialect optional when the model has no write-through entities, or document that they're required regardless and inert for value-object-only models.

Context

All three came up in a from-scratch TypeScript adopter (the smon system monitor). Happy to link the repo/config if useful.

[dmealing]

All three constraints are addressed on main — two of them fixed rather than documented.

1. metadata.root child rules are closed — documented. docs/features/extending-with-providers.md now carries a "Defining a custom top-level node type" section spelling out that registering the subtype is not enough: a provider must also registry.extend(TYPE_METADATA, SUBTYPE_ROOT, { childRules: [...] }) to license it as a permitted child, or the node fails to load with ERR_CHILD_NOT_ALLOWED.

2. Reference resolution only indexed object.* nodes — fixed in code (43d90ca1). The loader's symbol table is now keyed per node type, so a reference to any registered top-level node kind resolves under the same ADR-0042 package-local contract (FQN-exact, else the referrer's package, else root-level), and the unresolved-error message names the actual target kind. Object-target references are byte-identical — the whole conformance corpus is unchanged; the change is strictly enabling.

3. dbImport / dialect required for a DB-less project — fixed in code (43d90ca1). Both are now optional, so a value-object-only model no longer has to supply dead-but-mandatory placeholders.

The interim "Adopter constraints" doc block that described 2 and 3 as live limitations was removed when they were fixed, so the docs don't now describe constraints that no longer exist.

Shipped via PR #245. Unreleased — rides the next npm cut; see CHANGELOG.md [Unreleased].