perf(streams): avoid repeated growing-buffer prefix extraction in SSE decoding - #7676

Summary

Keep only the bounded six-code-unit prefix needed to recognize an SSE data: field instead of repeatedly extracting that prefix from a growing line. Update the prefix after accepted appends and reset it at each accepted line.

Preserve CR, LF and split-CRLF handling, UTF-8 decoding, byte accounting, event limits and translator-budget admission/release. Extend regressions for fragmented long lines, multilingual fields and failure cleanup.

Verification

• HEAD c7607ae0e41d971529ba05b731945b73b8735953 normally merges source 4771faba9eaf395e9ba58e83c185a1dee1b3e26f with dev@bd5c2ea906741a4201c6f35bb180e7a9a112a4fd. Both change-owned blobs are unchanged; root/GUI dependency inputs match dev.
• Official Bun 1.4.4, native Windows, CI=1: bun test --isolate --timeout 60000 tests/responses/sse-carriage-return.test.ts tests/responses/sse-decoder.test.ts — 25 passed / 0 failed / 277 assertions. This includes acceptance at exactly 32 MiB and rejection at one byte above the limit.
• Root TypeScript, Structure and diff checks passed. Repository privacy/file-size evaluators passed over both changed files; unrelated sparse-absent paths were not evaluated.
• Existing isolated frozen dependencies matched current inputs and installed manifests; no reinstall or live settings change. Source clean and owned processes/fixtures absent after validation.
• Source review checked successful-append ordering, line reset, delimiters, Unicode and unchanged admission/release behavior. Existing Structure contracts remain accurate. The operation-count regression distinguishes repeated growing-line prefix extraction; it does not establish total worst-case copying, process RSS or end-to-end throughput.
• Full-suite exception: focused regression and static checks were used alongside other worktrees. Complete-suite and native macOS/Linux runs, benchmarks and new upstream CI remain pending. Earlier source-head CI is historical, not current-merge success.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

Co-authored-by: luvs01 27862058+luvs01@users.noreply.github.com