📝 Add global logger API docs

docs/api/global-trace.md

@@ -0,0 +1,77 @@
+---
+name: global-trace
+group: api
+category: global
+update-time: 20260512
+description: Emit a trace-level record through the shared default logger shortcut.
+key-word:
+    - global
+    - trace
+    - default
+    - public
+---
+
+## Global-trace
+
+Emit a trace-level record through the shared default logger. This is the global convenience wrapper for `log(Level::Trace, ...)`.
+
+### Interface
+
+```moonbit
+pub fn trace(message : String, fields~ : Array[Field] = []) -> Unit {}
+```
+
+#### input
+
+- `message : String` - Trace message text.
+- `fields : Array[Field]` - Optional structured fields attached to the record.
+
+#### output
+
+- `Unit` - No return value. The record is handled through the shared default logger.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper delegates to `default_logger().trace(...)`.
+- Trace is the lowest built-in severity and is commonly disabled unless the shared minimum level is lowered.
+- The shared default target and console sink are used.
+- This helper favors convenience over explicit per-component logger ownership.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Quick Global Trace Output
+
+When a script or small app wants temporary low-level diagnostics:
+```moonbit
+set_default_min_level(Level::Trace)
+trace("entered sync loop")
+```
+
+In this example, the shared global path begins accepting trace records.
+
+#### When Attach Structured Trace Context
+
+When a global trace event should carry fields:
+```moonbit
+trace("cache probe", fields=[field("key", "user:42")])
+```
+
+In this example, the global helper still supports structured metadata.
+
+### Error Case
+
+e.g.:
+- If the shared minimum level is above `Trace`, the record is skipped.
+
+- If you need per-call target control, use an explicit `Logger` instead of this helper.
+
+### Notes
+
+1. Global trace logging is convenient for scripts but easy to overuse in larger systems.
+
+2. This helper always goes through the shared default logger path.
+