feat: refuse placeholder versions in the schema

core_version is a string the code states, null or keyed by build mode,
and the schema refuses Git, SVN, unknown, master, main, latest, dev and
HEAD, alone or annotated. version_basis is defined per kind with the
fields each one needs, and kind none requires a null version.

tests/test_version_basis.py reads a recorded fixture of forge answers
offline and fails on drift, holds every profile to a basis (tag versions
in describe form, aliases equal to their profile, none as null), checks
that a pin advance reads the version again or is refused, and that the
schema refuses placeholders.

scripts/profile_sync.py

@@ -2736,7 +2736,8 @@ def set_version_fields(text: str, version: object, basis: dict | None) -> str:
     if span is None:
         anchor = _top_block(lines, "upstream_commit") or _top_block(lines, "source_commit") \
             or _top_block(lines, "profiled_date") or _top_block(lines, "upstream") \
-            or _top_block(lines, "source")
+            or _top_block(lines, "source") or _top_block(lines, "alias_of") \
+            or _top_block(lines, "type")
         if anchor is None:
             raise YamlWriteError("core_version: no line to write it after")
         lines[anchor[1]:anchor[1]] = rendered