feat: first-draft preview-capable zosstorage

- CLI: add topology selection (-t/--topology), preview flags (--show/--report), and removable policy override (--allow-removable) (src/cli/args.rs)
- Config: built-in sensible defaults; deterministic overlays for logging, fstab, removable, topology (src/config/loader.rs)
- Device: discovery via /proc + /sys with include/exclude regex and removable policy (src/device/discovery.rs)
- Idempotency: detection via blkid; safe emptiness checks (src/idempotency/mod.rs)
- Partition: topology-driven planning (Single, DualIndependent, BtrfsRaid1, SsdHddBcachefs) (src/partition/plan.rs)
- FS: planning + creation (mkfs.vfat, mkfs.btrfs, bcachefs format) and UUID capture via blkid (src/fs/plan.rs)
- Orchestrator: pre-flight with preview JSON (disks, partition_plan, filesystems_planned, mount scheme). Skips emptiness in preview; supports stdout+file (src/orchestrator/run.rs)
- Util/Logging/Types/Errors: process execution, tracing, shared types (src/util/mod.rs, src/logging/mod.rs, src/types.rs, src/errors.rs)
- Docs: add README with exhaustive usage and preview JSON shape (README.md)

Builds and unit tests pass: discovery, util, idempotency helpers, and fs parser tests.

src/errors.rs

@@ -0,0 +1,56 @@
+//! Common error types and result alias for zosstorage.
+
+use thiserror::Error as ThisError;
+
+/// Top-level error for zosstorage covering configuration, validation,
+/// device discovery, partitioning, filesystem, mounting, reporting,
+/// and external tool invocation failures.
+#[derive(Debug, ThisError)]
+pub enum Error {
+    /// Invalid or malformed configuration input.
+    #[error("configuration error: {0}")]
+    Config(String),
+
+    /// Semantic validation failure.
+    #[error("validation error: {0}")]
+    Validation(String),
+
+    /// Errors related to device discovery and probing.
+    #[error("device discovery error: {0}")]
+    Device(String),
+
+    /// Partitioning or GPT manipulation failures.
+    #[error("partitioning error: {0}")]
+    Partition(String),
+
+    /// Filesystem creation or probing failures.
+    #[error("filesystem error: {0}")]
+    Filesystem(String),
+
+    /// Mount operation failures.
+    #[error("mount error: {0}")]
+    Mount(String),
+
+    /// State report construction or write failures.
+    #[error("report error: {0}")]
+    Report(String),
+
+    /// External system tool invocation failure with captured stderr.
+    #[error("external tool '{tool}' failed with status {status}: {stderr}")]
+    Tool {
+        tool: String,
+        status: i32,
+        stderr: String,
+    },
+
+    /// Placeholder for not-yet-implemented functionality (e.g., --force).
+    #[error("unimplemented: {0}")]
+    Unimplemented(&'static str),
+
+    /// Any other error wrapped with context.
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}
+
+/// Crate-wide result alias.
+pub type Result<T> = std::result::Result<T, Error>;