diff --git a/CHANGELOG.md b/CHANGELOG.md
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index 0000000..49a3700
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,11 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- Initial scaffold: module declaration, package skeleton, sentinel errors, CI workflow.
diff --git a/errors.go b/errors.go
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--- /dev/null
+++ b/errors.go
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+package blockdev
+
+import "errors"
+
+var (
+	// ErrMisaligned indicates an offset or length is not a multiple of BlockSize.
+	ErrMisaligned = errors.New("blockdev: offset or length not block-aligned")
+
+	// ErrOutOfBounds indicates a read or write extends beyond the device length.
+	ErrOutOfBounds = errors.New("blockdev: read/write beyond device length")
+
+	// ErrBadFormat indicates serialized data is malformed and cannot be deserialized.
+	ErrBadFormat = errors.New("blockdev: malformed serialized data")
+)