[r-bin]

for reasons I don't comprehend Evermore takes random numbers too serious and creates them in the middle of rendered frames.

Summary

During active display the PPU owns OAM and CGRAM. A CPU read of $2138/$213B returns the byte at the address the PPU is
fetching at that moment; the CPU's address still advances. snesrecomp always answers with the CPU's address.

 ppu_read($2138 / $213B)
-  return mem[cpu_address]
+  if rendering and (h, v) = beam at this access's own clock is not in a free window
+    return mem[ppu_fetch_address(h, v)]
+  return mem[cpu_address]
   cpu_address++

Rules, as MesenCE 2.2.1 has them:

• OAM, H ≤ 1020: the range phase's sprite, one per two dots from the priority-rotation start.
• OAM, later: the time phase, walking this line's in-range sprites backward from dot 270, two dots per sliver.
• CGRAM: free in H 0–87 and 1096–1363, on line 0, from the NMI line on, and in forced blank. Otherwise the backdrop
  while the sub screen has no layer enabled.

snes.c installs g_ppu_beam_hook, the beam at the access's own clock (from #241). It only answers on the master-clock
timeline (RtlApuUseMasterClock), so other games keep today's reads. SNESRECOMP_PPU_RENDER_READS=0 turns it off.

Evidence

Secret of Evermore seeds its RNG at reset from reads on lines 124 and 125, so one wrong byte changes the rest of the run.
Attract replay, WRAM/VRAM/CGRAM/OAM hashed at every NMI against MesenCE 2.2.1, on main e9c20732:

• Before (SNESRECOMP_PPU_RENDER_READS=0): WRAM parts at NMI 5998, native and interpreted.
  After: identical at all 8,592 NMIs, native and interpreted.

<test: tests/ppu/… reads $2138/$213B at chosen H/V and checks the fetched byte> ← to write

Merge Danger

Door: two-way. One switch turns it off, and it is inactive unless the master-clock timeline is on.

Blast Radius: narrow. Only games on the master-clock timeline that read $2138/$213B during active display.

Limits: checked against one game's reads only. CGRAM with sub-screen layers on is not modelled (keeps the CPU's address).
Each $2138 read during rendering walks the lines above the beam, which would cost a game that polls it.