build: eliminate all remaining compile warnings

Production (hand-fixed, semantics preserved with width-limited %.*s):
- persist.c (33 sites): workdir/name joins share the destination budget;
  WAL segment/snapshot paths via a WAL_SEG_WALPATH_MAX macro derived from
  seg_path's own declared size; trim loops use d_name's NAME_MAX (255) cap.
- config_repl.c (16 sites) and ha.c's config-hash twin: same half-split
  and fixed-literal-tail budgets, byte-identical path construction kept
  between ha.c and conf_save_config.
- main.c: certdir/key/cert paths leave room for their fixed suffixes.

Test suites (-Wno-format-truncation, rationale in-tree): the 68 remaining
sites are all snprintf(dir, 4096, "%s/<literal>") fixture scratch paths
under mkdtemp /tmp/xce_* dirs that never approach PATH_MAX -- gcc cannot
prove the tail fits, but every one is a false positive. Applied as a
compile_args declare_dependency on each suite's shared deps list.

Vendored c-capnproto: -Wno-switch (~40 exhaustive-dispatch false
positives), -Wno-unused-function (empty define_encode_function stub),
-Wno-format-overflow (255-byte uniontag + 3-byte prefix vs 256-byte buf,
unreachable schema lengths). Both the wrap checkout and the
packagefiles overlay patched identically.

Also: test_worker_launch_type_dispatch removes its fixed-path fake-xrun
/tmp fixture at exit (bug-924 /tmp hygiene; the mkdtemp shim can't see it).

Verified: fresh full recompile of every source = 0 warnings (gcc 13.3,
-Wall asan profile); meson test 134/134 green; no /tmp/xce_* residue.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/controller/ha.c

@@ -164,9 +164,13 @@ static int upgrade_materialization_read_live_config_sha256(cluster_t *c,
         return -ENOENT;
 
     /* effective_dir = workdir/name — must match conf_save_config's own path
-     * construction (config_repl.c) exactly, mirroring local_config_sha256. */
+     * construction (config_repl.c) exactly, mirroring local_config_sha256.
+     * Width-limited %.*s: workdir/name fit conf_dir so "/conf/<fname>" (fixed
+     * + 256-byte tail) provably fits path (4096+256) below. */
     char conf_dir[4096];
-    snprintf(conf_dir, sizeof(conf_dir), "%s/%s", mc->workdir, mc->name);
+    snprintf(conf_dir, sizeof(conf_dir), "%.*s/%.*s",
+             (int)(sizeof(conf_dir) / 2), mc->workdir,
+             (int)(sizeof(conf_dir) / 2 - 2), mc->name);
 
     char fname[256] = {0};
     int rc = conf_read_current_filename(conf_dir, fname, sizeof(fname));
@@ -174,7 +178,9 @@ static int upgrade_materialization_read_live_config_sha256(cluster_t *c,
         return -ENOENT; /* no local bytes yet — caller fences + retries */
 
     char path[4352];
-    snprintf(path, sizeof(path), "%s/conf/%s", conf_dir, fname);
+    snprintf(path, sizeof(path), "%.*s/conf/%.*s",
+             (int)(sizeof(path) - sizeof("/conf/") - 256), conf_dir, 256,
+             fname);
 
     uint8_t *bytes = NULL;
     size_t   blen  = 0;