Fixes #18119 (missed a spot in #16985)

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The bug

A {@const} that captures a let: slot prop throws ReferenceError: Cannot access '<name>' before initialization when the let: directive is on a slotted element (not on the component itself):

<!-- Parent.svelte -->
{#each data as row}
  <slot name="row" {row}></slot>
{/each}

<!-- Consumer.svelte -->
<Parent>
  <tr slot="row" let:row>
    {@const props = { row }}
    <td>{props.row.label}</td>
  </tr>
</Parent>

This is the same class of bug as #16585, but #16985 only fixed the variant where let: is on the component itself. The slotted-element variant still regresses.

Root cause

{@const} declarations are collected into state.consts, and Fragment emits them as [...state.let_directives, ...state.consts, ...state.init]. LetDirective declarations on a component are routed into let_directives (since #16985), so they correctly come first.

But in RegularElement, the let: declarations were pushed onto context.state.init:

// Let bindings first, they can be used on attributes
context.state.init.push(...lets);

Since init is emitted after consts, the generated code declared the {@const} derived before the let: derived it depends on:

const props = $.derived_safe_equal(() => ({ row: $.get(row) })); // ← references row
const row = $.derived_safe_equal(() => $$slotProps.row);          // ← declared after

In dev mode the {@const} derived is read eagerly ($.get(props)), so this hits the temporal dead zone and throws.

The fix

Push the element's let: declarations into let_directives instead of init, so they precede {@const} declarations, mirroring the component-level handling from #16985. They still come before the element's own init (and therefore before any attributes that use them), since Fragment emits let_directives before init.

Testing

Added runtime-legacy/samples/let-directive-and-const-tag-slotted (compiled with dev: true, which is where the eager read surfaces the error). It fails before this change with Cannot access 'thing' before initialization and passes after.

Full runtime-legacy, runtime-runes, server-side-rendering, hydration, snapshot, css, validator and print suites pass.