fix(lib): a malformed select in a resreq fails the parse instead of being dropped (bug-1887)

The primary_resreq action ignored parse_resreq_primary()'s result, so a
primary whose select (or rusage) clause failed to parse was accepted
without that clause: select[type==] or select[mem>] matched every host.
A malformed clause now fails the whole resreq, and parse_resreq() returns
the clause's own message.

A select naming an unregistered resource is still dropped, not an
error: parse_select() reports it as -ENOENT (new select_context_t
.unknown_resource) and the action lets it pass, because jobs rely on it
-- the scheduler's own "type == any" default among them when no "type"
resource is registered (making it fatal failed 36 scheduler suites).

Test: test_resreq_primary_select_error_fails_parse RED before (all
accepted), GREEN after.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/include/resreq.h

@@ -191,6 +191,7 @@ struct select_context_t_ {
     int                err_pos;
     int                has_error;
     char               errmsg[256]; /* fixed-size — no alloc/free in grammar actions */
+    int                unknown_resource; /* bug-1887: the error was an unknown resource name */
     int                depth;       /* current nesting depth — guard vs SELECT_MAX_DEPTH */
     xce_select_node_t* root;        /* result root set by grammar top-level action */
 };