Move m68k fork out of vendor/ and rename it to copperline-m68k

[LinuxJedi]

Problem

Offline/vendored builds (openSUSE OBS, and anyone using cargo vendor) failed with:

failed to load checksum `.cargo-checksum.json` of m68k v0.1.5 ... No such file or directory

Distro packaging vendors crates.io dependencies into a directory named vendor/ and emits:

[source.crates-io]
replace-with = "vendored-sources"
[source.vendored-sources]
directory = "vendor"

Cargo then treats every subdirectory of vendor/ as a vendored registry crate and requires a .cargo-checksum.json for each. Our committed path crate lived in vendor/m68k with no checksum file, so the offline build aborted.

Fix

• Relocate the fork vendor/m68k -> crates/m68k, so it no longer collides with the packaging vendor/ directory.
• Rename the package m68k -> copperline-m68k. The name no longer matches the upstream crates.io m68k crate, and the dependency carries no version requirement, so no tool, [patch], or future workspace member can substitute the unmodified upstream core for this fork.
• Keep the importable library name as m68k via [lib] name, and alias the dependency back with package = "copperline-m68k", so no source use m68k::... changes were needed.
• Update .gitignore, VENDOR.md, and the remaining vendor/m68k path references in docs and one source comment.

The 127 committed *.bin test fixtures (force-added past the global *.bin ignore) were carried over to the new path as renames; none were lost.

Verification

• cargo build --release, cargo test --release (1194 passed, 0 failed), cargo clippy, cargo fmt --check: all clean.
• The fork's own test suite still compiles/runs under the new lib name.
• Reproduced the OBS scenario locally and confirmed it now succeeds:

cargo vendor vendor-check
cargo build --release --offline \
  --config 'source.crates-io.replace-with="vendored-sources"' \
  --config 'source.vendored-sources.directory="vendor-check"'

copperline-m68k builds from the local path while every crates.io dep resolves from the vendored directory.

Closes #57