Fix bugs found by clang-tidy triage; ratchet warning budget to zero (#3047)

Real bugs fixed:
- manager.c: kill_server/stop_server checked fscanf() != EOF, so malformed
  pid file content left pid uninitialized and passed it to kill(). Parse
  with fgets + ss_parse_int and require pid > 0.
- utils.c: get_default_conf() crashed on strlen(NULL) when HOME is unset
  (e.g. daemons started by init) and left a dangling static pointer after
  free(), a latent use-after-free on repeated calls. Use a static buffer
  and fall back to the system config when HOME is missing.
- local.c: in UDP_ONLY mode, start_ss_local_server() passed uninitialized
  listen_ctx.fd to the library callback. Initialize it to -1.

Improvements:
- Replace rand()/srand(time(NULL)) upstream-server selection in ss-local,
  ss-redir and ss-tunnel with libsodium randombytes_uniform(): unbiased,
  unpredictable, and no seeding required (cert-msc30/msc32).
- Mark FATAL() noreturn so both the compiler and analyzers understand
  control flow (removes a family of analyzer false positives).
- Drop two dead stores (manager.c restore_sigchld, udprelay.c src_addr_len).

All remaining findings were verified as false positives and carry NOLINT
comments with rationale (uthash macro internals, symmetric back-pointer
cleanup, analyzer-invisible postconditions). clang-tidy-18 on Linux now
reports zero warnings, so MAX_WARNINGS drops from 29 to 0.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/tests.yml

@@ -83,11 +83,12 @@ jobs:
   clang-tidy:
     runs-on: ubuntu-latest
     env:
-      # Ratchet: number of pre-existing clang-tidy warnings (see .clang-tidy),
+      # Ratchet: number of known clang-tidy warnings (see .clang-tidy),
       # counted with clang-tidy-18 (pinned below so runner upgrades don't move
-      # the number). CI fails if the count goes UP; lower this value as
-      # findings get fixed.
-      MAX_WARNINGS: 29
+      # the number). All baseline findings have been fixed or triaged
+      # (false positives carry NOLINT comments with rationale), so any new
+      # warning fails CI.
+      MAX_WARNINGS: 0
     steps:
       - uses: actions/checkout@v6
         with: