add depth configuration parity with the reference driver

parse_depth_mode returned NEURAL for any unrecognised string, so a typo
silently changed depth quality, and NEURAL_PLUS and NEURAL_LIGHT could not be
selected at all. All six modes are now reachable and an unknown name exits
non-zero listing the accepted values.

Parsing moves into Config::from_env() and main reads the configuration before
init_dora_node(), so a bad value fails with its own message instead of after
the node has registered with the dataflow.

Add ZED_CONFIDENCE_THRESHOLD (60) and ZED_TEXTURE_CONFIDENCE_THRESHOLD (100),
which were never set, and change the defaults to 1080 / 200 mm / 600 mm to
match the reference driver perception was tuned against. The resolved settings
are logged at startup.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7g2cBJ5e3r15ZbJ9WZn8c

src/main.cc

@@ -91,9 +91,11 @@ int64_t host_timestamp_ns() {
 } // namespace
 
 int main() {
-    auto dora_node = init_dora_node();
-
+    // Read the environment before joining the dataflow, so a bad configuration
+    // fails with its own message rather than after the node has registered.
     auto config = zed::Config::from_env();
+
+    auto dora_node = init_dora_node();
     zed::Camera camera(config);
     camera.open();
     camera.warmup();