refactor: derive the milestone bounds from the registry

The highest supported milestone was spelled out in six places across four
files: range(1, 7) and default="m6" in test_cli.py, the same pair in
test_milestones_dosboxx.py, and through=6 in both registry.py and suite.py.
Registering M7 meant finding all six, and missing one failed silently.

Derive MAX_MILESTONE and MILESTONES from CASES instead, and move the mN
selector parser to registry.milestone_number so the pytest module stops
carrying its own copy. Adding cases for a new milestone is now enough for
ferro-test to accept --through/--only for it.

No behaviour change: MAX_MILESTONE evaluates to 6, ferro-test still advertises
{m1..m6} with default m6, and the case snapshot is unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BScg8CF1sAAM2zVHAu5zvW

tools/tests/test_milestones_dosboxx.py

@@ -6,23 +6,13 @@ import warnings
 import pytest
 
 from ferrolang_vm.dosboxx import SuiteRun, run_suite
-from ferrolang_vm.registry import all_cases
+from ferrolang_vm.registry import MAX_MILESTONE, all_cases, milestone_number
 from ferrolang_vm.suite import Case
 
-
-def _number(name: str) -> int:
-    if not name.startswith("m") or not name[1:].isdigit():
-        raise ValueError(f"invalid milestone: {name}")
-    value = int(name[1:])
-    if value not in range(1, 7):
-        raise ValueError(f"unsupported milestone: {name}")
-    return value
-
-
 ONLY = os.environ.get("FERRO_TEST_ONLY")
 CASES = all_cases(
-    through=_number(os.environ.get("FERRO_TEST_THROUGH", "m6")),
-    only=_number(ONLY) if ONLY else None,
+    through=milestone_number(os.environ.get("FERRO_TEST_THROUGH", f"m{MAX_MILESTONE}")),
+    only=milestone_number(ONLY) if ONLY else None,
 )