Cluster G of the #643 sweep — 3 fixes in one file, all making the test
suite Windows-compatible without touching production:

1) `test_project_install_profile_no_packages_dir` — skipif(win32). The
   test monkeypatches `sys.executable` to `"/usr/local/bin/python"` and
   then expects `mm_binary_origin == "system"`. On Windows `Path()`
   normalizes that string to backslashes, defeating the production
   `startswith("/usr/local/bin/")` check in `_detect_mm_binary_origin`.
   The fixture is POSIX-only by construction; rewriting it for Windows
   would need a Windows-flavored "system Python" path which has no
   single canonical answer (`C:\Python313\python.exe`? `py.exe`?).

2) `test_origin_system_paths` — skipif(win32). Same root cause: the
   test iterates over hardcoded POSIX paths (`/usr/bin/python3`,
   `/opt/homebrew/bin/python3.13`, etc.) and asserts `system`
   classification. None of these exist on Windows, and the prefix-match
   logic uses POSIX literals.

3) `_write_md` helper — `newline=""`. The helper is used by
   `test_collect_seed_scale_counts_md_only_and_sums_bytes` which
   asserts `total == len("hello world\n") + 100 == 112`. On Windows
   `Path.write_text` translates `\n` → `\r\n` by default (universal
   newlines), making the file 1 byte longer and the assertion fail
   `113 == 112`. Adding `newline=""` disables translation; production
   `_collect_seed_scale` already reads raw byte counts, so no
   production change is needed.

Mirrors PR #713's bundling pattern (multiple skipif sites in one PR
when they share the same POSIX-only rationale) and PR #714's bundling
pattern (helper-level fix sweeping the test sites that depend on it).

macOS: 262/262 pass. Lint clean.

Co-Authored-By: Claude <[email protected]>