Docs: regenerate performance tables for 4.0 #578

This was generated by AI during a pre-4.0 audit of main at 9cd88e7.

docs/introduction/performance.md:62-112 is the 3.x snapshot measured at 630de77; :316 says so. Since then #557 moved C6 by about 28%, and #561 and the perf issue change G6/G7/G8. The 4.0 notes paragraph does not mention #561.

Run just bench-report on the final pre-release main (after the code issues in the 4.0 milestone land) and regenerate the tables and the 4.0 notes.