📝 Add global logger API docs

docs/api/global-debug.md

@@ -0,0 +1,77 @@
+---
+name: global-debug
+group: api
+category: global
+update-time: 20260512
+description: Emit a debug-level record through the shared default logger shortcut.
+key-word:
+    - global
+    - debug
+    - default
+    - public
+---
+
+## Global-debug
+
+Emit a debug-level record through the shared default logger. This is the global convenience wrapper for `log(Level::Debug, ...)`.
+
+### Interface
+
+```moonbit
+pub fn debug(message : String, fields~ : Array[Field] = []) -> Unit {}
+```
+
+#### input
+
+- `message : String` - Debug 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().debug(...)`.
+- The record uses the current shared minimum level and target configuration.
+- Debug output is typically intended for development and troubleshooting.
+- This helper is best suited to small apps or code paths that intentionally rely on the shared logger.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Simple Global Debug Logging
+
+When code wants quick diagnostics without wiring a logger variable:
+```moonbit
+set_default_min_level(Level::Debug)
+debug("cache refreshed")
+```
+
+In this example, the shared global path starts accepting debug records.
+
+#### When Attach Structured Debug Context
+
+When a debug event should include machine-readable detail:
+```moonbit
+debug("session loaded", fields=[field("user_id", "42")])
+```
+
+In this example, the global shortcut still supports structured fields.
+
+### Error Case
+
+e.g.:
+- If the shared minimum level is above `Debug`, the record is skipped.
+
+- If different modules need distinct targets or sinks, a shared global debug path may be too coarse.
+
+### Notes
+
+1. Prefer explicit loggers once application logging becomes subsystem-specific.
+
+2. Use `set_default_min_level(...)` before expecting global debug output to appear.
+