feat(xce-harness-component): relocate xce_test into standalone harness component

Move xce_test's 15 production sources + headers from src/controller/testsuite/
into a new src/harness/ component with its own meson.build, and strip every
now-dangling controller-side reference (11 controller_unit_test_src entries,
4 standalone test-binary blocks, 11 controller_test.c extern/RUN_TEST lines)
so the controller build is never left broken.

Fixes a broken relative-symlink bridge (xce_test_controller_link) added to
keep two controller-embedded tests (test_test_cli.c, test_xcetest_process_entry.c)
finding the relocated xce_test binary until task 2.1 moves them too.

Discovered and documented in design.md: four controller-domain regression
tests call functions directly from four of xce_test's own files, requiring
controller_unit_test to permanently recompile them from src/harness/ — a
real dependency neither the original discovery nor design audit caught.

controller_unit_test: 1228 Tests 0 Failures 0 Ignored (verified via the
Unity binary's own printed summary line, not just meson's aggregate count).

Task 1.1 of xce-harness-component.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

src/harness/rcfile.c

@@ -0,0 +1,80 @@
+#define _GNU_SOURCE
+#include "rcfile.h"
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* Suffixes appended to the resolved base directory to form the rc-file
+ * path -- kept as two separate constants (rather than one shared tail)
+ * since the XDG_CONFIG_HOME branch is already rooted at a "config home"
+ * directory while the HOME fallback still needs the "/.config" segment. */
+#define RCFILE_XDG_SUFFIX  "/xce_test/rc"
+#define RCFILE_HOME_SUFFIX "/.config/xce_test/rc"
+
+char* rcfile_resolve_path(void)
+{
+    const char* xdg_config_home = getenv("XDG_CONFIG_HOME");
+    if (xdg_config_home && *xdg_config_home) {
+        size_t len = strlen(xdg_config_home) + strlen(RCFILE_XDG_SUFFIX) + 1;
+        char*  path = (char*)malloc(len);
+        if (!path)
+            return NULL;
+        snprintf(path, len, "%s%s", xdg_config_home, RCFILE_XDG_SUFFIX);
+        return path;
+    }
+
+    const char* home = getenv("HOME");
+    if (home && *home) {
+        size_t len = strlen(home) + strlen(RCFILE_HOME_SUFFIX) + 1;
+        char*  path = (char*)malloc(len);
+        if (!path)
+            return NULL;
+        snprintf(path, len, "%s%s", home, RCFILE_HOME_SUFFIX);
+        return path;
+    }
+
+    return NULL;
+}
+
+int rcfile_load(const char* path, int (*runner)(char* line))
+{
+    if (!path)
+        return 0;
+
+    FILE* f = fopen(path, "r");
+    if (!f) {
+        if (errno == ENOENT)
+            return 0; /* missing rc file: silent no-op success */
+        fprintf(stderr, "xce_test: warning: could not open rc file %s: %s\n",
+                path, strerror(errno));
+        return 0; /* present but unreadable: non-fatal warning */
+    }
+
+    int     rc   = 0;
+    char*   line = NULL;
+    size_t  cap  = 0;
+    ssize_t n;
+    while ((n = getline(&line, &cap, f)) != -1) {
+        if (n > 0 && line[n - 1] == '\n')
+            line[n - 1] = '\0';
+        rc = runner(line);
+        if (rc != 0)
+            break;
+    }
+
+    /* fopen() succeeds on a directory on this platform -- the failure only
+     * surfaces on the first read (getline() returns -1 with errno==EISDIR
+     * and ferror(f) set, feof(f) NOT set). Treat that the same as an open
+     * failure: one non-fatal stderr warning, no runner calls attributed to
+     * it (rc is still 0 here since no line was ever successfully read). */
+    if (rc == 0 && ferror(f)) {
+        fprintf(stderr, "xce_test: warning: could not read rc file %s: %s\n",
+                path, strerror(errno));
+    }
+
+    free(line);
+    fclose(f);
+    return rc;
+}