refactor(scope): bind the parent link once and gate re-linking behind the binding

setScopeParent could re-link any key from anywhere, leaving the
blank-session-only recompose rule entirely to caller discipline. The
relation now binds once — a second bind throws — and re-linking exists
only on the ScopeParentBinding returned to the original binder, the
private-capability shape the package conventions prescribe for a
single-caller operation. The preset roster keeps each composed agent's
binding in a WeakMap keyed by the agent, making it the sole authority
that can move an agent between standing compositions; the blank-session
contract itself stays with the gateway, which alone can see what a
session logged.

packages/core/scope/README.md

@@ -2,12 +2,12 @@
 
 English | [中文](README.zh.md)
 
-Scoped registration primitive. `createScope(ctx, key)` creates a tagged Cordis context whose backing fiber owns every registration made through it. `scopeOf(ctx)` reads the tag, and `scopeTarget(base, key)` routes scoped events to listeners with the same key while leaving unscoped listeners global. Keys form an optional parent chain (`setScopeParent`): registration views inherit DOWN it — a child scope sees its ancestors' layers, nearest shadowing farthest — and event admission extends UP it — a listener tagged with an ancestor receives a descendant key's events, never the reverse. The agent loop creates one scope per live agent and an agent preset's standing mount is a parent scope over its agents, but the mechanism is key-agnostic so lower-level packages can use it without depending on either.
+Scoped registration primitive. `createScope(ctx, key)` creates a tagged Cordis context whose backing fiber owns every registration made through it. `scopeOf(ctx)` reads the tag, and `scopeTarget(base, key)` routes scoped events to listeners with the same key while leaving unscoped listeners global. Keys form an optional parent chain (`bindScopeParent`): registration views inherit DOWN it — a child scope sees its ancestors' layers, nearest shadowing farthest — and event admission extends UP it — a listener tagged with an ancestor receives a descendant key's events, never the reverse. The agent loop creates one scope per live agent and an agent preset's standing mount is a parent scope over its agents, but the mechanism is key-agnostic so lower-level packages can use it without depending on either.
 
 ## Public API
 
-- `createScope(ctx: Context, key: ScopeKey, options?): Scope` Mint a scope under `ctx`'s fiber. Usable synchronously (effect collection is uid-gated; service resolution falls through to the minting plugin's dependency surface). The typed, same-process key is trusted; an inactive minting context still fails through Cordis (`INACTIVE_EFFECT`). `options.parent` records the enclosing scope via `setScopeParent` before the scope is usable.
-- `setScopeParent(key, parent)` / `scopeParentOf(key)` / `scopeChainOf(key)` The parent relation behind both chain directions. Ordinarily written once at mint; re-linking an existing key is the blank-session recompose operation, valid only while nothing produced under the old parent is retained (the caller's contract — this relation cannot see what a session logged). A link closing a cycle throws. `scopeChainOf` returns `[key, parent, …]` nearest-first.
+- `createScope(ctx: Context, key: ScopeKey, options?): Scope` Mint a scope under `ctx`'s fiber. Usable synchronously (effect collection is uid-gated; service resolution falls through to the minting plugin's dependency surface). The typed, same-process key is trusted; an inactive minting context still fails through Cordis (`INACTIVE_EFFECT`). `options.parent` binds the enclosing scope via `bindScopeParent` before the scope is usable; the binding stays internal.
+- `bindScopeParent(key, parent): ScopeParentBinding` / `scopeParentOf(key)` / `scopeChainOf(key)` The parent relation behind both chain directions. Binding is once: a key that already has a parent throws, and only the returned binding's `rebind(parent)` may re-link it — the blank-session recompose operation, valid only while nothing produced under the old parent is retained (the holder's contract — this relation cannot see what a session logged). Both the bind and every rebind reject a link closing a cycle. `scopeChainOf` returns `[key, parent, …]` nearest-first.
 - `Scope.ctx` The tagged context: registrations through it are scope-visible AND scope-lifetime. Derived contexts (an `extend`, a fiber mounted under it) inherit the tag; nested scopes shadow (nearest tag wins).
 - `Scope.rawDispose` The EXACT Cordis disposer for the backing fiber — a composite (generator) effect yields THIS function to nest the scope's teardown at that yield position (Cordis dedupes nested effects by function identity; yielding a wrapper leaves the scope disposing as a concurrent sibling).
 - `Scope.dispose(): Promise<void>` Idempotent, shared quiescence boundary for every registration made through the scope. Racing/repeat calls await the same teardown, including when `rawDispose` invoked the underlying single-shot Cordis disposer first.