📝 Add filter predicate API docs

docs/api/target-is.md

@@ -0,0 +1,79 @@
+---
+name: target-is
+group: api
+category: filtering
+update-time: 20260512
+description: Create a reusable record predicate that matches a single exact target.
+key-word:
+    - target
+    - filter
+    - predicate
+    - public
+---
+
+## Target-is
+
+Create a `RecordPredicate` that returns `true` only when a record target exactly matches the expected string. Use it when one specific logical source should pass through a filter.
+
+### Interface
+
+```moonbit
+pub fn target_is(target : String) -> RecordPredicate {}
+```
+
+#### input
+
+- `target : String` - Exact target value expected from the record.
+
+#### output
+
+- `RecordPredicate` - Predicate that matches records whose `rec.target` equals `target`.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Matching uses direct string equality.
+- This helper does not perform prefix or fuzzy matching.
+- It is useful for isolating one subsystem when targets are intentionally stable.
+- Combine it with level predicates when a single target needs stricter routing rules.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Keep One Exact Target
+
+When only one service target should pass:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_filter(target_is("gateway.http"))
+```
+
+In this example, records from sibling targets such as `gateway.ws` are excluded.
+
+#### When Route A Single Target To A Sink
+
+When a split path needs an exact match:
+```moonbit
+let predicate = all_of([
+  target_is("audit"),
+  level_at_least(Level::Info),
+])
+```
+
+In this example, the filter remains predictable because it does not rely on naming prefixes.
+
+### Error Case
+
+e.g.:
+- If `target` is empty, only records whose target is also an empty string will match.
+
+- If record targets are generated dynamically, exact matching may be too strict and lead to no matches.
+
+### Notes
+
+1. Prefer `target_has_prefix(...)` when targets are hierarchical and you want subtree matching.
+
+2. Exact target filters are easiest to maintain when target naming is stable across the project.
+