[codex] fix simson Vendetta audio balance

[jtmiki]

Summary

• Restore the Simpsons/Vendetta K053260 auxiliary FM mix behavior for the simson core
• Keep the ADPCM saturation path available by parameter, but use the 94364a5-style ADPCM accumulation for simson
• Restore the 33n RC pole and set simson audio gain to 0.97 to avoid startup peak LED flashes

Root Cause

Vendetta sounded muted/muffled in the 2026-06-11 9c688b8 build compared with the 2025-08-09 94364a5 build. The relevant behavioral difference was that K053260 AUX/FM mixing became gated by mode[3:2]. For simson/Vendetta this can drop the FM contribution, making the output sound muted. The PR keeps the newer behavior as the default for shared jt053260 users and opts simson into the older always-mixed AUX behavior.

Validation

• jtframe mem simson -l from cores/simson/ver/vendetta: generated g0 = 8'h7C (0.97)
• jtcore simson -mrq: PASS, 0 errors, 410 warnings
• Tested on La MiSTer TV during development: Vendetta audio no longer sounded muted; The Simpsons had no warning LED flashes or clipping at gain 0.95, then gain was raised to 0.97 for final test build