docs(action): the anchor-name failure prints the name

An anchor whose name carries a control character survives lexing and fails at conversion when a collection is used as a key, and the yaml library's message prints the anchor name after a fixed prefix. The boundary bullet listing what a parse failure can print now names that case beside the syntax error and the unresolved alias.

Author: Vivswan

docs/operate/layering.md

@@ -273,7 +273,7 @@ That guarantee covers the fold alone. The per-layer validation prints the same m
   - `branches` wildcard entries, by name with the scalar declared where a mapping belongs, the widest echo in the set: `branches[0].protection.required_status_checks: the wildcard entry "release/*" declares protection.required_status_checks as "strict", but on a wildcard rule it must be a mapping`.
   - `environments` entries, by name: `environments[0].deployment_branch_policies: the "prod" entry declares deployment_branch_policies, so it must also declare deployment_branch_policy`.
   - `actions.selected_actions` repeats the declared `allowed_actions`, which has already passed its enum, so only `all`, `local_only`, or `selected` can appear there.
-- A YAML syntax error, quoting the offending source line with a caret under the column; an unresolved alias names the alias instead.
+- A YAML syntax error, quoting the offending source line with a caret under the column; an unresolved alias names the alias instead, and an anchor whose name carries whitespace or a control character prints that name after `Anchor must not contain whitespace or control characters:`.
 - A YAML parser warning (an unresolved tag, an unknown directive, an ambiguous anchor, a collection used as a key) prints nothing: the parser runs with its warnings off, so such a document parses silently to the same object it always did, and only a parse failure reaches the log, printing what the bullet above describes.
 
 A merge-mode log can therefore show your settings file's structure and, through these messages, a value from it: treat it like any log that prints a parse error for a file the runner holds.