📝 Add filter predicate API docs

docs/api/level-at-least.md

@@ -0,0 +1,79 @@
+---
+name: level-at-least
+group: api
+category: filtering
+update-time: 20260512
+description: Create a reusable record predicate that keeps records at or above a minimum level.
+key-word:
+    - level
+    - filter
+    - predicate
+    - public
+---
+
+## Level-at-least
+
+Create a `RecordPredicate` that returns `true` when a record level is greater than or equal to a minimum threshold. This is the main helper for severity-based filtering.
+
+### Interface
+
+```moonbit
+pub fn level_at_least(min_level : Level) -> RecordPredicate {}
+```
+
+#### input
+
+- `min_level : Level` - The lowest level that should remain enabled.
+
+#### output
+
+- `RecordPredicate` - Predicate that keeps records whose `priority()` is at least the given minimum.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Matching is based on `Level::priority()`, not string comparison.
+- Records at the exact same level as `min_level` are included.
+- This helper is useful when a sink or child logger needs a stricter threshold than the parent logger.
+- It composes cleanly with `all_of(...)` and `any_of(...)`.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Keep Warnings And Errors
+
+When a logger should keep only important records:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_filter(level_at_least(Level::Warn))
+```
+
+In this example, `Warn` and `Error` records continue through the filter.
+
+#### When Combine With Target Routing
+
+When both severity and namespace matter:
+```moonbit
+let predicate = all_of([
+  target_has_prefix("service.api"),
+  level_at_least(Level::Info),
+])
+```
+
+In this example, the predicate can be reused by multiple loggers or sinks.
+
+### Error Case
+
+e.g.:
+- If `min_level` is `Level::Trace`, the predicate effectively allows all normal log records.
+
+- If `min_level` is higher than the record level, the predicate returns `false` without modifying the record.
+
+### Notes
+
+1. Use this helper when you want filtering logic to remain explicit instead of embedding level checks inline.
+
+2. This predicate is separate from a logger's own `min_level`, so both can be combined.
+