refactor: move data_dir override from env var to [server] TOML field

Reverts the NUMA_DATA_DIR env var added in the previous commit and
replaces it with a [server] data_dir TOML field. Numa already has a
well-developed config system; adding a parallel env-var mechanism
for a single knob was wrong.

The principle: TOML is for application behavior configuration. Env
vars are for bootstrap values (HOME, SUDO_USER to discover paths
before config loads) and standard ecosystem conventions (RUST_LOG).
data_dir is neither — it's an app knob, so it belongs in the TOML.

Changes:
- lib.rs::data_dir() reverts to the platform-specific fallback only
- config.rs adds `data_dir: Option<PathBuf>` to ServerConfig
- main.rs resolves config.server.data_dir with fallback to
  numa::data_dir() and passes it to build_tls_config, then stores the
  resolved path on ctx.data_dir for downstream consumers
- tls.rs::build_tls_config takes `data_dir: &Path` as an explicit
  parameter instead of calling crate::data_dir() behind the caller's
  back. regenerate_tls and dot.rs self_signed_tls now pass
  &ctx.data_dir, honoring whatever path the config resolved to
- tests/integration.sh Suite 6 uses `data_dir = "$NUMA_DATA"` in its
  test TOML instead of the NUMA_DATA_DIR env var prefix
- numa.toml gains a commented-out data_dir example

No behavior change for existing production deployments (the default
path is unchanged). Test harness is now fully config-driven, and
containerized deploys can override data_dir via mount+config without
needing env var injection.

127/127 unit tests pass, Suite 6 passes end-to-end.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -204,10 +204,23 @@ async fn main() -> numa::Result<()> {
 
     let forwarding_rules = system_dns.forwarding_rules;
 
+    // Resolve data_dir from config, falling back to the platform default.
+    // Used for TLS CA storage below and stored on ServerCtx for runtime use.
+    let resolved_data_dir = config
+        .server
+        .data_dir
+        .clone()
+        .unwrap_or_else(numa::data_dir);
+
     // Build initial TLS config before ServerCtx (so ArcSwap is ready at construction)
     let initial_tls = if config.proxy.enabled && config.proxy.tls_port > 0 {
         let service_names = service_store.names();
-        match numa::tls::build_tls_config(&config.proxy.tld, &service_names, Vec::new()) {
+        match numa::tls::build_tls_config(
+            &config.proxy.tld,
+            &service_names,
+            Vec::new(),
+            &resolved_data_dir,
+        ) {
             Ok(tls_config) => Some(ArcSwap::from(tls_config)),
             Err(e) => {
                 log::warn!("TLS setup failed, HTTPS proxy disabled: {}", e);
@@ -248,7 +261,7 @@ async fn main() -> numa::Result<()> {
         config_path: resolved_config_path,
         config_found,
         config_dir: numa::config_dir(),
-        data_dir: numa::data_dir(),
+        data_dir: resolved_data_dir,
         tls_config: initial_tls,
         upstream_mode: resolved_mode,
         root_hints,