Milestone 2 phase B, etapa 1 - microdrive write support

Milestone 2, phase B, etapa 1: microdrive write support (SAVE), on top of the existing read support. SAVE/DIR/LOAD round-trip tested working on real MEGA65 R6 hardware, including saving and reloading a program. The R3 build compiles clean (positive timing margins, 0 unrouted nets) but has not yet been tested on real R3 hardware - if you have one, please try it and report back.

See INSTRUCTIONS.md (attached below) for exactly where each file goes on the SD card, and CHANGELOG.md / DECISIONES.md in the repository for the full milestone-by-milestone history (including the two-round debugging story behind this release - a strobe-timing bug and a counter-width overflow, the second one found and simulated by an external model after internal static analysis stalled).

Included

• QL4M65-CoreQL-M2024_r6.cor - the compiled core for MEGA65 R6 (tested)
• QL4M65-CoreQL-M2024_r3.cor - the compiled core for MEGA65 R3 (untested on real hardware)
• main.rom - Minerva 1.98, plain (no QL-SD driver - this core doesn't implement QL-SD yet)
• back.rom - Toolkit II (TK2)
• blank.mdv - a blank, correctly-formatted microdrive test image
• INSTRUCTIONS.md - installation instructions

Minerva and TK2 are GPL/GNU-licensed and redistributed here on that basis. For other QL ROMs, see https://sinclairql.net/djw/qlrom/index.html.

Known limitations of this milestone: microdrive writes land in the FPGA's own buffer only - there's no way yet to flush them back to the SD card's .mdv file (next up), one drive (mdv1_) only, 128 KB RAM (no expansion yet), QL-native CPU speed only, PAL video only.