phrasing and routing: a raw newline is not JSON (V-537)

Sixty of the failures in the 2026-08-05 temperature sweep were one error,
`phraser: model output starts as JSON but does not parse`, all of them in the
reply family and two of them in all twelve runs. The write-up read that as
truncation. It is not: no run hit the token cap.

The string rule in both grammars was `[^"\\]`, which admits a literal
newline. A model that wants two lines writes one, the generation satisfies the
grammar, and json.Unmarshal then rejects it with "invalid character '\n' in
string literal". The object starts with "{", so it came back as errBrokenJSON
and the reply was an empty string. The router's rule also admitted `"\\" .`,
so \q satisfied it and failed to parse the same way.

Both string rules are now llama.cpp's own json.gbnf class: the control range is
out and the escape alternatives are exact. Verified against the resident model
on 8899 — llama-server accepts both grammars and both still emit what they did.

escapeRawControls is the second line, for NoGrammar and for a remote server that
ignores a grammar: a reply whose only fault is a raw newline is readable, so it
is read rather than dropped.

internal/phraser/grammar_test.go

@@ -90,8 +90,13 @@ func TestNoGrammarConfigDisablesIt(t *testing.T) {
 // The grammar's string rule must accept any codepoint, not just ASCII. Replies
 // are Russian: an ASCII-only class would constrain the model into empty replies.
 func TestGrammarStringRuleIsNotASCIIOnly(t *testing.T) {
-	if !strings.Contains(responseGrammar, `([^"\\] | "\\" ["\\/bfnrt])`) {
-		t.Error("string rule is not the any-codepoint-except-quote-and-backslash class; Cyrillic replies would be impossible")
+	if !strings.Contains(responseGrammar, `[^"\\\x00-\x1F]`) {
+		t.Error("string rule is not the any-codepoint-except-quote-backslash-and-controls class; Cyrillic replies would be impossible")
+	}
+	// The control range must be out (Vikunja #537): a raw newline inside a JSON
+	// string is not JSON, and the model wrote one whenever it wanted two lines.
+	if strings.Contains(responseGrammar, `([^"\\] |`) {
+		t.Error("string rule still admits raw control characters; a multi-line reply will fail to parse")
 	}
 }