feat(xce-test-rc-file): task 1.2 rc-file line-loading via pluggable runner

Adds rcfile_load(path, runner) to rcfile.c/rcfile.h: reads lines with
getline() (no fixed-size buffer, so a long line like a multi-peer
`set masters ...` can never be silently truncated or split), strips the
trailing newline, invokes the caller-supplied runner once per line, and
stops at the first nonzero runner return, propagating that value. A missing
rc file is a silent no-op; any other open failure prints one stderr warning
and returns 0.

Found during GREEN: fopen() succeeds on a directory path on this platform,
so the EISDIR failure only surfaces on the first getline() call (ferror(f)
set) -- added a post-loop ferror() check to catch it (bug-1716).

5 new unit tests (11 total with task 1.1): ENOENT-silent, per-line-in-order
with newline stripped, stop-on-nonzero with exact propagation, non-ENOENT
open-failure warning, and a >4KB single line proving no truncation/split.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgYRU3zDTqNGV5Jk8aiKLg

src/controller/testsuite/rcfile.h

@@ -1,15 +1,17 @@
 #pragma once
 /*
- * rcfile.h -- xce_test startup rc-file path resolution (xce-test-rc-file
- * spec, task 1.1).
+ * rcfile.h -- xce_test startup rc-file path resolution and line-loading
+ * (xce-test-rc-file spec, tasks 1.1/1.2).
  *
  * A standalone, dependency-light module following the same pattern as
  * configcompare.c/h -- no dependency on test.c internals, linked into both
  * the xce_test executable and controller_unit_test.
  *
- * This task implements path resolution only (rcfile_resolve_path()).
- * Line-loading (rcfile_load()) is a later task in the same spec and will
- * extend this file, not replace it.
+ * Task 1.1 implemented path resolution (rcfile_resolve_path()). Task 1.2
+ * adds line-loading (rcfile_load()) with a pluggable runner callback.
+ * Wiring rcfile_load() into xce_test's own startup/main() is a later task
+ * in the same spec (2.2) and is out of scope here -- this module has no
+ * knowledge of xce_test commands, contexts, or client state.
  */
 
 /* Resolves the rc-file path per the XDG Base Directory Specification:
@@ -18,3 +20,20 @@
  * a heap string the caller must free(), or NULL if neither variable yields
  * a usable base directory. */
 char* rcfile_resolve_path(void);
+
+/* Opens `path` and calls `runner(line)` once per line (trailing newline
+ * stripped), stopping early if `runner` returns nonzero. Lines are read via
+ * getline() with no length cap -- a line is never truncated or split
+ * regardless of length.
+ *
+ * Returns:
+ *   0                    - path is NULL, the file does not exist (ENOENT),
+ *                           the file could not be opened for another reason
+ *                           (a warning is printed to stderr; non-fatal), or
+ *                           every line ran and every runner call returned 0.
+ *   nonzero (== runner's) - the runner requested an abort; no further lines
+ *                           are read.
+ *
+ * Preconditions: `runner` is non-NULL when `path` is non-NULL.
+ * Postconditions: the file handle is always closed before returning. */
+int rcfile_load(const char* path, int (*runner)(char* line));