docs(react-islands): drop the write-during-render guard

Implemented and removed. A render-phase flag cleared on a microtask is
still set when React runs effects, so islands writing from an effect —
the documented correct pattern — would throw. The flag also cannot be
set for an island's own re-renders, so real violations pass silently.

Detecting React's render phase reliably needs React internals, which is
not acceptable in a shipped framework. React already reports the real
hazard, and the getSnapshot caching requirement covers the loop case.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/superpowers/specs/2026-08-18-react-islands-design.md

@@ -127,9 +127,9 @@ record that subscribers and devtools depend on.
 Write → action → store notifies → snapshot changes → island re-renders. This terminates cleanly
 **provided writes never occur during render**. Writes belong in event handlers or effects.
 
-This rule is enforced in dev (see Error handling), not merely documented. It is also the reason
+This rule is documented rather than machine-enforced — see "Dropped: the write-during-render
-this shape was chosen over generated `bind:` sugar: the cycle stays visible in the author's own
+guard" below. It is also the reason this shape was chosen over generated `bind:` sugar: the
-code rather than being hidden in generated glue.
+cycle stays visible in the author's own code rather than being hidden in generated glue.
 
 ## Compiler and bundler changes
 
@@ -189,12 +189,38 @@ point.
 | Island bundle fails to load | Placeholder remains, warning logged; page stays functional because everything else was server-rendered |
 | Non-serializable props | Compile-time `WRN-ISLAND-PROPS` |
 | Unknown store name | Dev: throw, listing available store names. Prod: warn, return undefined |
-| Action fired during render | Dev: throw with a targeted message pointing at the handler/effect rule (React's own warning is too generic to diagnose quickly) |
+| Action fired during render | Left to React. See "Dropped: the write-during-render guard" below. |
 | Cleanup throws on unmount | Caught and logged; navigation must not break |
 
 Islands failing **locally** is the most valuable property of this model: a crashed chart leaves
 the rest of the page working.
 
+## Dropped: the write-during-render guard
+
+The design originally called for a dev-only guard that threw when an island
+called a store action during render. It was implemented, then removed: the
+mechanism is unreliable in both directions.
+
+- **False positives.** React runs effects before a queued microtask drains, so a
+  render-phase flag cleared on a microtask is still set inside `useEffect`. An
+  island writing from an effect — the documented correct pattern — would throw.
+- **False negatives.** The flag can only be set from the error boundary`s
+  render. When an island updates its own state, only the island re-renders, so
+  the flag is never set and a genuine write-during-render passes silently.
+
+There is no reliable public API for detecting React`s render phase; doing it
+properly requires React internals, which is not acceptable in a shipped
+framework.
+
+React already covers the real hazard: writing during render that notifies
+subscribers produces "Cannot update a component while rendering a different
+component", and the infinite-loop case is caught by the `getSnapshot` caching
+requirement handled in the store bridge. The custom guard added false positives
+without covering anything React misses.
+
+The author-facing rule still stands and is still documented — it is simply not
+machine-enforced.
+
 ## Testing
 
 ### Compiler unit tests