host: relations-as-posts slice 3 — typed relations (target-type constraint enforced)

A relation's declares-anchor IS its target-type constraint: is-a/subtype-of (anchored
by type) require a type object; tagged (anchored by tag) a tag; related (no anchor) any
post. host/blog--valid-object?(kind, other) = other ∈ the relation's candidate pool — the
SAME set the picker offers — and relate-submit now enforces it (invalid target = silent
no-op). The picker never offers an invalid target, so this guards crafted/API requests:
the jump from candidate set to an enforced relation schema. A new typed relation needs
only a relation-post + a '<TargetType> declares <rel>' edge.

host/blog-relate! (direct/seed) stays unvalidated — validation is a handler boundary
(the seed writes 'X is-a relation', and relation isn't under type).

conformance 291/291 (+4: valid-object? accepts types/tags/any, relate-submit creates the
edge for a type object and no-ops for a non-type).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plans/relations-as-posts.md

@@ -63,12 +63,17 @@ the relation's object-end declaration from the anchor**, which includes the root
   the picker O(1)-perform and cut the suspend/resume churn. Subject-end declarations + a
   proper relation-subtype closure (when relations get subtyped) also belong here.
 
-### Slice 3 — typed relations (target-type constraints)
-- A declaration carries a **target-type constraint**: the *other* end must be (an instance
-  of) some type. `is-a`'s object must be a type; a hypothetical `wrote`'s object must be a
-  `Work`. Validation on relate (and on save) = `is-a?` against the constraint. This is the
-  jump from "candidate set" to a real relation schema. Picker candidates and validation
-  read the *same* constraint.
+### Slice 3 — typed relations (target-type constraints) — DONE
+- The declaration's `declares`-anchor IS the target-type constraint: `is-a`/`subtype-of`
+  (anchored by `type`) require a type object; `tagged` (anchored by `tag`) a tag. A new
+  `wrote` relation needs only a `Work declares wrote` edge — fully data-driven.
+- `host/blog--valid-object?(kind, other)` = `other ∈ candidate-pool(kind)` — the SAME set
+  the picker offers, so picker and validation agree by construction. `relate-submit` now
+  enforces it (an invalid target is a silent no-op, like the other guards); `related`
+  (no anchor) accepts any post. The picker never offers an invalid target, so this guards
+  crafted/API requests — the jump from "candidate set" to an enforced relation schema.
+- NOTE: `host/blog-relate!` (direct/seed) stays UNVALIDATED — the seed needs to write
+  `X is-a relation` where `relation` isn't under `type`. Validation is a *handler* boundary.
 
 ### Slice 4 — type algebra
 Types are posts + `subtype-of` is a partial order ⇒ a **lattice**, and `is-a?` is transitive