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/types.rs

@@ -0,0 +1,170 @@
+//! Typed configuration schema for zosstorage.
+//!
+//! Mirrors docs in [docs/SCHEMA.md](docs/SCHEMA.md) and is loaded/validated by
+//! [fn load_and_merge()](src/config/loader.rs:1) and [fn validate()](src/config/loader.rs:1).
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LoggingConfig {
+    /// Log level: "error" | "warn" | "info" | "debug"
+    pub level: String,     // default "info"
+    /// When true, also log to /run/zosstorage/zosstorage.log
+    pub to_file: bool,     // default false
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DeviceSelection {
+    /// Regex patterns to include device paths (Rust regex).
+    pub include_patterns: Vec<String>,
+    /// Regex patterns to exclude device paths.
+    pub exclude_patterns: Vec<String>,
+    /// Whether to include removable devices (future).
+    pub allow_removable: bool,
+    /// Minimum device size (GiB) to consider eligible.
+    pub min_size_gib: u64,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum Topology {
+    /// Single eligible disk; btrfs on remainder.
+    Single,
+    /// Two eligible disks; independent btrfs on each data partition.
+    DualIndependent,
+    /// SSD + HDD; bcachefs with SSD cache/promote and HDD backing.
+    SsdHddBcachefs,
+    /// Optional mirrored btrfs across two disks when explicitly requested.
+    BtrfsRaid1,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct BiosBootSpec {
+    /// Whether to create a tiny BIOS boot partition.
+    pub enabled: bool,
+    /// Size in MiB (default 1).
+    pub size_mib: u64,
+    /// GPT partition name (e.g., "zosboot").
+    pub gpt_name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct EspSpec {
+    /// ESP size in MiB (default 512).
+    pub size_mib: u64,
+    /// Filesystem label for ESP (ZOSBOOT).
+    pub label: String,
+    /// GPT partition name (e.g., "zosboot").
+    pub gpt_name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DataSpec {
+    /// GPT partition name for data (e.g., "zosdata").
+    pub gpt_name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CacheSpec {
+    /// GPT partition name for cache partitions (e.g., "zoscache").
+    pub gpt_name: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Partitioning {
+    /// Alignment in MiB (default 1 MiB).
+    pub alignment_mib: u64,
+    /// Abort if any target disk is not empty (default true).
+    pub require_empty_disks: bool,
+    /// BIOS boot partition spec.
+    pub bios_boot: BiosBootSpec,
+    /// ESP partition spec.
+    pub esp: EspSpec,
+    /// Data partition spec.
+    pub data: DataSpec,
+    /// Cache partition spec (only in ssd_hdd_bcachefs).
+    pub cache: CacheSpec,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct BtrfsOptions {
+    /// Filesystem label (ZOSDATA).
+    pub label: String,
+    /// Compression string (e.g., "zstd:3").
+    pub compression: String,
+    /// "none" | "raid1"
+    pub raid_profile: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct BcachefsOptions {
+    /// Filesystem label (ZOSDATA).
+    pub label: String,
+    /// "promote" | "writeback" (if supported).
+    pub cache_mode: String,
+    /// Compression algorithm (e.g., "zstd").
+    pub compression: String,
+    /// Checksum algorithm (e.g., "crc32c").
+    pub checksum: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct VfatOptions {
+    /// Filesystem label (ZOSBOOT).
+    pub label: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct FsOptions {
+    /// btrfs tuning.
+    pub btrfs: BtrfsOptions,
+    /// bcachefs tuning.
+    pub bcachefs: BcachefsOptions,
+    /// vfat tuning for ESP.
+    pub vfat: VfatOptions,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum MountSchemeKind {
+    /// Mount under /var/cache/<UUID>
+    PerUuid,
+    /// Reserved for future custom mappings.
+    Custom,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MountScheme {
+    /// Base directory (default: /var/cache).
+    pub base_dir: String,
+    /// Scheme kind (PerUuid | Custom (reserved)).
+    pub scheme: MountSchemeKind,
+    /// When true, write /etc/fstab entries (disabled by default).
+    pub fstab_enabled: bool,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ReportOptions {
+    /// Path for JSON state report (default: /run/zosstorage/state.json).
+    pub path: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Config {
+    /// Schema version (start at 1).
+    pub version: u32,
+    /// Logging configuration.
+    pub logging: LoggingConfig,
+    /// Device selection and filtering rules.
+    pub device_selection: DeviceSelection,
+    /// Desired topology mode.
+    pub topology: Topology,
+    /// Partitioning parameters.
+    pub partitioning: Partitioning,
+    /// Filesystem options and tuning.
+    pub filesystem: FsOptions,
+    /// Mount scheme and fstab policy.
+    pub mount: MountScheme,
+    /// Report output configuration.
+    pub report: ReportOptions,
+}