docs(mister): name the device-under-test repository, now that it exists - #434

The spec described a "sibling repository (`RustyNES_MiSTer`)" with no link,
because at the time there was nothing to link to. There is now:
https://github.com/doublegate/RustyNES_MiSTer, private.

Two additions beyond the URL, both about things a reader of this document
needs and could not previously get.

THE CROSS-REPOSITORY COUPLING IS NAMED, BECAUSE BREAKING IT IS SILENT

`tb/checkpoint.h` over there reimplements `Observable::encode` from here, and
`tb/checkpoint_selftest.cpp` asserts it against the same hardcoded vector
`the_wire_encoding_is_pinned_to_a_fixed_vector` pins on this side. That pairing
is the entire guard against the top-ranked risk at this rung: a packing
disagreement between the two halves produces a hash mismatch indistinguishable
from a wrong DUT, and it would be debugged as one.

So the document now states plainly that changing `encode` here without
changing `checkpoint.h` there breaks co-simulation in the way that is hardest
to diagnose, and that the selftest is the first thing to run after touching
either. A coupling that spans two repositories has nothing mechanical watching
it, which is exactly why it has to be written where the person editing the
encoding will read it.

Verified rather than asserted before writing it down: both files were parsed
and their pinned vectors compared byte for byte.

  rust: 16 bytes  08 07 06 05 04 03 02 01 f5 c5 23 81 a9 01 05 00
  cpp : 16 bytes  08 07 06 05 04 03 02 01 f5 c5 23 81 a9 01 05 00

and the selftest re-run against merged main, all five checks green.

Worth recording how that nearly went wrong: the first comparison was a `grep
-A4` over the Rust array, which returned three bytes -- rustfmt had reformatted
the literal one element per line, so the context window was too small. Three
bytes against sixteen reads as a mismatch. The published comparison parses both
literals properly instead.

THE LICENCE HEDGE IN ADR 0037 IS RESOLVED, AND INVERTED

ADR 0037 recorded that a GPL-2.0-ONLY file anywhere in the MiSTer framework's
sys/ would force the RTL to GPL-2.0-or-later, and that the tabulation had to
happen before any RTL existed. It has: all 57 files read and classified.

There is no GPL-2.0-only file. Four are GPL-3.0-OR-LATER, and one of them is
hps_io.sv, which is how a core receives a ROM and reaches the OSD -- no core
functions without it. GPL-2.0-or-later combines upward into GPL-3;
GPL-3.0-or-later does not reduce. The combined bitstream must therefore be
GPL-3.0-or-later, which is already this project's licence.

The hedge is inverted by the evidence rather than confirmed by it, and the
decision is made with zero lines of RTL written rather than ten thousand --
which was the entire point of ordering the audit first.

Docs only. No source, no manifests, no core.