📝 Add logger write API docs

docs/api/logger-trace.md

@@ -0,0 +1,77 @@
+---
+name: logger-trace
+group: api
+category: logging
+update-time: 20260512
+description: Emit a trace-level record through the logger using the lowest built-in severity shortcut.
+key-word:
+    - logger
+    - trace
+    - sync
+    - public
+---
+
+## Logger-trace
+
+Emit a trace-level record through the synchronous logger. This is the convenience wrapper for `log(Level::Trace, ...)`.
+
+### Interface
+
+```moonbit
+pub fn[S : Sink] Logger::trace(self : Logger[S], message : String, fields~ : Array[Field] = []) -> Unit {}
+```
+
+#### input
+
+- `self : Logger[S]` - Logger that should emit the trace record.
+- `message : String` - Trace message text.
+- `fields : Array[Field]` - Optional structured fields attached to the record.
+
+#### output
+
+- `Unit` - No return value. The record is handled according to the logger threshold and sink pipeline.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper delegates to `log(Level::Trace, ...)`.
+- Trace is the lowest built-in severity and is often disabled in production.
+- Per-call target override is not exposed here; use `log(...)` if that is needed.
+- Context, filtering, patching, and queue wrappers still apply through the logger sink chain.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Low-level Execution Tracing
+
+When fine-grained flow diagnostics are useful:
+```moonbit
+logger.trace("entered reconciliation step")
+```
+
+In this example, trace intent is explicit and concise.
+
+#### When Attach Structured Trace Context
+
+When a trace event should include fields:
+```moonbit
+logger.trace("cache probe", fields=[field("key", "user:42")])
+```
+
+In this example, trace output stays lightweight while still carrying structured detail.
+
+### Error Case
+
+e.g.:
+- If the logger minimum level is above `Trace`, the call returns without writing a record.
+
+- If a per-call target override is required, this helper is too narrow and `log(...)` should be used instead.
+
+### Notes
+
+1. Prefer this helper when trace intent is clearer than a raw `log(Level::Trace, ...)` call.
+
+2. Very verbose trace logging can become expensive even in a synchronous pipeline.
+