PHLEGM v4 NES mapper compatibility checkpoint

- Generalized NES removable-media loader from ines-nrom to ines.
- Added mapper 1 / MMC1 PRG banking, CHR banking, serial register writes, and mapper-controlled mirroring.
- Added mapper 3 / CNROM switchable 8 KiB CHR banking and legacy bus-conflict handling.
- Retained mapper 0 / NROM.
- Routed CPU writes to mapper registers before cartridge ROM write protection discards them.
- Routed PPU pattern-table reads/writes through the active cartridge mapper.
- Increased resident cartridge capacity for banked MMC1/CNROM images.
- State/session format advanced to 6 so mapper-resident bank/register state restores exactly. Formats 4/5 remain readable for mapper-0 snapshots.
- Added raw iNES header, payload/trailing-byte, and reset-vector diagnostics at cartridge insertion.
- NES-NTSC machine definition now advertises mappers 0,1,3.

Target cases: 1942 (mapper 0), Blaster Master (mapper 1), Arkanoid (mapper 3).
