Summary

Fixes #15627

Scripts rendered through Astro.slots.render('default') with set:html were being hoisted to the top of the slot output instead of rendering at their original position. This regression was introduced by PR #15147, which changed renderScript() from returning a plain HTML string to returning a RenderInstruction object.

Root Cause

In renderSlotToString(), RenderInstruction objects (including scripts) were collected into a separate instructions array. When the SlotString was stringified in stringifyChunk(), all instructions were rendered before the content string — causing scripts to be hoisted to the top of the slot output regardless of their original position.

Fix

Uses positional placeholders (<!--astro:script:N-->) embedded in the content string at each script's original position. The actual script instructions are stored in a scriptInstructions map on SlotString. At stringification time, placeholders are replaced with the real script HTML.

This preserves:

• Script position — the placeholder marks where the script should appear
• Script deduplication — still goes through stringifyChunk which checks renderedScripts
• Unused Fragment slot behavior — if a slot is unused, script instructions are never stringified (PR Fixes scripts not rendering with unused Fragment slots #15147's fix preserved)
• Server island slot scripts — updated server-islands.ts to resolve script placeholders in island responses

Files Changed

• packages/astro/src/runtime/server/render/slot.ts — Added scriptInstructions field to SlotString, script instructions now use positional placeholders in content string
• packages/astro/src/runtime/server/render/common.ts — Replace script placeholders with stringified content during SlotString processing
• packages/astro/src/runtime/server/render/server-islands.ts — Resolve script placeholders when serializing slot content for island responses

Test plan

• New test: "Scripts rendered via Astro.slots.render preserve their position" — verifies script is inside <li>, not hoisted before <ol>
• Existing test passes: "Scripts in Fragment slots are processed when another slot is unused"
• Existing test passes: "includes script from slotted component in island response"
• All 35 script + server-island tests pass
• Build passes