Phase 3, Task 11: Make NextJsNextDataRewriter fragment-safe

[aram356]

Parent: #563

Summary

`NextJsNextDataRewriter` assumes it receives complete text node content in a single `rewrite()` call. Under the streaming `HtmlRewriterAdapter`, lol_html may split text nodes across chunk boundaries, causing silent missed rewrites on large `NEXT_DATA` payloads.

Changes

• Accumulate text fragments in `NextJsNextDataRewriter` until `TextChunk::last_in_text_node()` is true
• Only perform the rewrite on the complete accumulated text
• Add regression test with small chunk size that splits a `NEXT_DATA` payload mid-string

File: `crates/trusted-server-core/src/integrations/nextjs/script_rewriter.rs`

Plan

See `docs/superpowers/specs/2026-03-25-streaming-response-design.md` — Phase 3