📝 Add record patch API docs

docs/api/identity-patch.md

@@ -0,0 +1,76 @@
+---
+name: identity-patch
+group: api
+category: patching
+update-time: 20260512
+description: Create a reusable record patch that returns records unchanged.
+key-word:
+    - patch
+    - transform
+    - record
+    - public
+---
+
+## Identity-patch
+
+Create a `RecordPatch` that returns the input record unchanged. This helper is useful as a neutral default, a placeholder in configuration, or a composition baseline.
+
+### Interface
+
+```moonbit
+pub fn identity_patch() -> RecordPatch {}
+```
+
+#### output
+
+- `RecordPatch` - Patch function that returns the original record without modification.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The returned patch is a pure pass-through transformation.
+- It does not allocate new fields or rewrite existing properties.
+- This helper is useful when a patch slot must be provided but no mutation is currently needed.
+- It also works as a safe baseline inside `compose_patches(...)` pipelines.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Neutral Patch
+
+When a logger pipeline expects a patch but no rewrite is required yet:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_patch(identity_patch())
+```
+
+In this example, the logger shape stays consistent while records remain unchanged.
+
+#### When Build Patches Conditionally
+
+When a branch may or may not enable a real patch:
+```moonbit
+let patch = if enable_redaction {
+  redact_field("token")
+} else {
+  identity_patch()
+}
+```
+
+In this example, caller code can still treat the result as a normal `RecordPatch`.
+
+### Error Case
+
+e.g.:
+- `identity_patch()` has no failure path; it simply returns the original record.
+
+- If a caller expects visible transformation, using this patch will intentionally produce none.
+
+### Notes
+
+1. Use this helper when you want a no-op patch with explicit intent instead of a custom inline closure.
+
+2. It is especially useful in conditional or generated patch pipelines.
+