fix: increment signal versions when discarding forks

[Rich-Harris]

Extracts part of #17335 and builds on it. If a derived is read for the first time inside a fork, we don't want to set derived.v (unless it has no dependencies, in which case it can be cached forever), but we do want the derived to correctly update when it's subsequently read outside the fork. The easiest way to achieve that is to always treat a derived as dirty if its value is UNINITIALIZED on read, which is what currently happens on this PR.

Though maybe the correct fix looks more like this? Not 100% sure just yet Edit: yes, it is, added tests that confirm it

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 134ebe53a2..e2f10f2101 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -972,6 +972,10 @@ export function fork(fn) {
 			await settled;
 		},
 		discard: () => {
+			for (var [source] of batch.current) {
+				source.wv = increment_write_version();
+			}
+
 			if (!committed && batches.has(batch)) {
 				batches.delete(batch);
 				batch.discard();

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