fix: preserve fragment data identity across snapshot updates (#68)

* fix: preserve fragment data identity across snapshot updates

`reconcile({ key: "__id", merge: true })` only preserves identity by
walking the existing tree in place when the current store value is
wrappable. solid-js/store's `reconcile` (modifiers.ts) early-returns
the new value as-is when it isn't, so reconciling against `undefined`
silently produces a fresh top-level reference and the merge contract
goes away.

The result observer was pre-clearing `data` to `undefined` inside the
same batch that subsequently applied the reconciled snapshot, so that
contract was being defeated on every Relay update. Identity-sensitive
consumers — most prominently `<Show keyed when={data()}>` — re-mounted
their subtree on every snapshot tick, including pure field updates
such as a reaction toggle or a polled count delta. In a real fediverse
timeline (hackers.pub `/feed`, ~25 cards × multiple Kobalte primitives
per card), this dominated CPU and heap during interaction (~+10 MB/s
sustained vs. ~0 MB/s when idle on the same route).

Restructure the observer so each branch owns its full state transition
and no shared pre-clears defeat the reconcile:

  - "ok"     — clear `error`, set `pending` to false, reconcile `data`.
  - "error"  — set `data` to `undefined`, set `error`, set `pending`.
  - "loading" — no case. Leave `data` / `error` / `pending` as-is for
    stale-while-revalidate behaviour, matching `createLazyLoadQuery`'s
    observer in the same package.

Behaviour is otherwise unchanged. Observers never saw the intermediate
pre-clear state — every mutation is inside the same `batch()` — so the
fix is invisible to anything except identity comparisons across snapshot
boundaries, which the reconcile contract is exactly there to keep
stable.

Assisted-by: Claude Code:claude-opus-4-7

* chore: clean up comments

* fix: handle loading fragment state properly

* chore: add changeset

---------

Co-authored-by: Iha Shin <me@xiniha.dev>

Author: nyanrus

.changeset/common-clocks-grab.md

@@ -0,0 +1,5 @@
+---
+"solid-relay": patch
+---
+
+fix: preserve fragment data identity across snapshot updates

src/primitives/createFragment.ts

@@ -132,9 +132,6 @@ export function createFragmentInternal<
 		next(res) {
 			queueMicrotask(() => {
 				batch(() => {
-					setResult("data", undefined);
-					setResult("error", undefined);
-
 					switch (res.state) {
 						case "ok":
 							setResult("error", undefined);
@@ -146,6 +143,11 @@ export function createFragmentInternal<
 							setResult("error", res.error);
 							setResult("pending", false);
 							break;
+						case "loading":
+							setResult("data", undefined);
+							setResult("error", undefined);
+							setResult("pending", true);
+							break;
 					}
 				});
 			});