fix: support bind: in {#each} with $derived($store) expressions

[FrankFMY]

Closes #13569

Problem

When using {#each} with bind: on items from a $derived expression that wraps a store (e.g. {#each $derived($store) as item}), mutations through bind: silently fail — neither the input nor any expression reading the derived updates.

This happens because the compiler doesn't detect the store subscription hidden behind the derived binding:

1. store_to_invalidate is empty — the each block's mutation handler never calls invalidate_store, so the store is never notified of changes
2. EACH_ITEM_IMMUTABLE is set — items use strict === equality instead of safe_equals, so same-reference object mutations aren't detected
3. The derived uses strict === equality — even if the store were invalidated, the derived returns the same array reference and === says "no change", preventing propagation to subscribers outside the each block

Fix

Compiler (EachBlock.js):

• Added find_store_sub_name() helper that walks an AST expression tree to find store subscription bindings
• Extended uses_store detection: when a dependency is a derived binding, traces through its initial (init expression) to find store subscriptions
• Extended store_to_invalidate detection: same tracing for derived bindings
• As a result, EACH_ITEM_IMMUTABLE is correctly not set when the expression depends on a store through a derived

Compiler (VariableDeclaration.js):

• When generating code for $derived(expr) where expr depends on a store subscription, the compiler now emits $.derived_safe_equal() instead of $.derived(). This uses object-aware equality (safe_equals) rather than strict reference equality (===), so same-reference mutations are detected and propagated to all subscribers (including expressions outside the each block like <p>{items[0].text}</p>)

Test

Added each-bind-derived-store test that verifies:

• bind:value on items from {#each $derived($store) as item} works
• Changes propagate both inside the each block and to a <p> tag reading from the derived outside the each block

[changeset-bot]

🦋 Changeset detected

Latest commit: 3624db9

The changes in this PR will be included in the next version bump.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17637-svelte.vercel.app/

[dummdidumm]

Thanks - marking as draft because I'm not so sure anymore that we should actually fix this (nor is this a complete fix likely, since it can appear in other situations, too)