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>

tests/integration.sh

@@ -542,8 +542,9 @@ else
     PROXY_HTTPS_PORT=8443
     NUMA_DATA=/tmp/numa-integration-data
 
-    # Fresh data dir so we generate a fresh CA for this suite — NUMA_DATA_DIR
-    # env var lets numa write under $TMPDIR instead of /usr/local/var/numa.
+    # Fresh data dir so we generate a fresh CA for this suite. Path is set
+    # via [server] data_dir in the TOML below, not an env var — numa treats
+    # its config file as the single source of truth for all knobs.
     rm -rf "$NUMA_DATA"
     mkdir -p "$NUMA_DATA"
 
@@ -551,6 +552,7 @@ else
 [server]
 bind_addr = "127.0.0.1:$PORT"
 api_port = $API_PORT
+data_dir = "$NUMA_DATA"
 
 [upstream]
 mode = "forward"
@@ -582,7 +584,7 @@ value = "10.0.0.1"
 ttl = 60
 CONF
 
-    NUMA_DATA_DIR="$NUMA_DATA" RUST_LOG=info "$BINARY" "$CONFIG" > "$LOG" 2>&1 &
+    RUST_LOG=info "$BINARY" "$CONFIG" > "$LOG" 2>&1 &
     NUMA_PID=$!
     sleep 4