[CarlosBakerUAi]

Summary

RUN_PYTHON expands to the interpreter path, and that path may contain spaces.
Unquoted, sh word-splits it, so on such a checkout every make target fails and
ensure-venv reports the wrong cause:

/bin/sh: /Volumes/Some Drive/repo/.venv/bin/python: not found
ERROR: OpenMontage requires Python 3.10+.
Current interpreter is unavailable: /Volumes/Some Drive/repo/.venv/bin/python

The interpreter is fine and already the right version. Nothing about the
message points at the space, so the natural response is to go install a Python
that was never the problem. make setup, make test, make preflight,
make demo and the rest are all unusable until you work it out.

Spaces in a checkout path are not exotic: an external volume, a synced folder,
or any user directory with a space in the name will do it.

Related issue

Not filed separately; happy to open one if you would prefer that first.

Changes

• Quote every command-position use of $(RUN_PYTHON) in the Makefile: the
  PIP recipe variable, recipe line starts, uses after ||, and the one
  inside a $$(...) command substitution.
• Display-only uses inside echo strings are deliberately left unquoted, since
  quoting them would just put quote marks in the printed output.
• No behavior change on paths without spaces.

Testing

New tests/tools/test_makefile_run_python_quoting.py, two tests:

1. A static check that no command-position $(RUN_PYTHON) is unquoted.
2. An end-to-end check that copies the Makefile into a temporary directory
   whose name contains a space, points VIRTUAL_ENV at it, and runs
   make ensure-venv for real.

Both fail on current main and pass with this change. Test 2 reproduces the
exact word-split error above.

Full suite on macOS: 965 passed, 11 skipped (963 before this PR, plus the two
tests added here).

Checklist

• The change is focused on a single logical concern.
• I ran the relevant tests locally (make test).
• I updated docs/README if behavior or usage changed. (No user-facing
  behavior or usage change; the same commands simply work now.)
• No unrelated files (build artifacts, local config) are included in the diff.

[qinghuanandejiangshi]

Reviewed the focused Makefile change and its regression coverage. I found no blocking issues.

I verified git diff --check, inspected every $(RUN_PYTHON) command-position use, and independently ran the test's static matcher against the proposed Makefile (zero unquoted command-position matches). The quoting is applied consistently to recipe starts, the PIP variable, the || fallback, and the nested command substitution, while display-only uses remain unchanged.

I could not exercise the GNU Make end-to-end test on this Windows host because make is not installed here; the test correctly marks that environment as skipped. The author's macOS end-to-end run covers the shell behavior this patch targets.

[calesthio]

Thank you for the focused Makefile quoting repair. Reviewed every changed command-position use of RUN_PYTHON, including PIP, the fallback command and nested substitution, against docs/PR_REVIEW_GUIDE.md. A clean integration with current main passed the static regression (1 passed); the GNU Make end-to-end case was correctly skipped because this Windows environment lacks make. The shell quoting is applied consistently without changing the display-only text, and the author's reported native run supplies the platform-specific execution evidence. No blocking issue found in this diff.