fix: close controller-peers-env-fallback validation gaps (bug-1909..1912)

- bug-1912: xce_parse_peers() ignores whitespace around entries and skips
  blank ones, matching xceadmin's resolve_controllers(), so "a=h:1, b=h:2"
  names the second candidate "b" (5 libxce tests + regression case).
- bug-1911: xcecd frees the -l log dir before the -h / --version returns
  (regression case: no LeakSanitizer report, exit 0).
- bug-1910: both version scripts build the apiserver with -mod=vendor
  (as the meson build does) instead of -mod=readonly, so they no longer
  need a Go module cache.
- bug-1909: test-version-startup-consistency.sh accepts exactly the
  dirty-content-fingerprint pairing ('<sha>-<8 hex>' in --version with
  '<sha>-dirty' in the startup log), with self-test cases (e)/(f).

Both version scripts now exit 0 on a dirty tree with XCE_CONTROLLER_PEERS
set; regression script 35/35; xce-build all: C 127/0, scheduler 16/0,
xceadmin 364 passed, no ASAN reports.

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

src/include/xce/cluster.h

@@ -129,7 +129,10 @@ void xce_reconnect_fini(xce_reconnect_ctx_t* rctx);
 /**
  * xce_parse_peers() — parse "--peers name=host:port,..." into cfg->candidates[].
  *
- * Each token may be "name=host:port" or bare "host:port".
+ * Each token may be "name=host:port" or bare "host:port". Whitespace around
+ * each comma-separated token is ignored and tokens that are blank after
+ * trimming are skipped (they do not use a MAX_HA_CANDIDATES slot), matching
+ * xceadmin's resolve_controllers() (bug-1912).
  * The name field is stored when present; it allows multiple daemons on the
  * same host to identify themselves by logical name rather than IP address.
  *