Recursion guard and static klammer checking

A klammer that reaches itself, directly or through a cycle, expanded
until the C++ stack was exhausted: the process died from SIGSEGV with no
message and no location. The former limit guarded only the top-level
fixed-point iteration, never the descent through klammer application. A
depth guard now raises a recursion error naming the klammer and where it
was applied. The same loop's termination test moves from "the katom list
stopped growing" to "a pass applied no klammer", since a klammer whose
body expands to nothing is a reduction that adds no katoms; exceeding the
round limit is now an error rather than a message followed by rendering a
document with live klammers still in it.

ktext --check locates every klammer application written in a document or
in a klammer body and checks name existence, argument count, option
names, and target coverage without applying anything, reporting all
problems at once. This is possible because Klammertext has no catcodes:
katom structure is fixed when a file is read, so a klammer body has a
determinate shape before it is expanded. The check therefore reaches what
the engine cannot -- the branch of a @cond that is not selected, and
bodies a given render never enters.

@cond's set of truth values is an open language question, so its meaning
is unchanged here; an unrecognized predicate now warns, giving its value
and location.

tst/ gains recursion_test.sh (7 cases) and check_test.sh (19 cases), and
this snapshot's test Makefile is generated from the shipped suite list so
the two cannot drift apart.

(from dev c27e63802406)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

mac/error.h

@@ -76,6 +76,17 @@ public:
         : Error("environment", description, locator, do_justify) {};
 };
 
+// Klammer application nested deeper than the engine's limit.  Raised by the
+// depth guard in Machine::apply_klammer(); without it a klammer that applies
+// itself (directly or through a cycle) exhausts the C++ stack and the process
+// dies with SIGSEGV and no diagnostic at all.
+class Recursion_error : public Error {
+public:
+    explicit Recursion_error(
+        const std::string& description, const Locator& locator=Locator(), bool do_justify=true)
+        : Error("recursion", description, locator, do_justify) {};
+};
+
 class Internal_error : public Error {
 public:
     explicit Internal_error(