slave (M4): add datadir inspection and lifecycle checks

cmd/slave/README.md

@@ -115,6 +115,69 @@ The running devnet config works the same way:
 
 prints `hash: 0x5ad443efb7cf5246a3d1bbc1734bd02bf3a5d83bedeccfcfe707d0ebee03780d`.
 
+### `slave inspect`
+
+Read-only and config-free: scan `--datadir` for shard chaindb directories
+(`shard-0x{full_shard_id}/`), open each in read-only mode, and print the stored
+minor genesis block and chain head. A shard that cannot be opened, read, or
+validated is reported without aborting the others, and the exit status is
+non-zero if any shard failed. A running slave holds its chaindb locks (each shard
+then reports `resource temporarily unavailable`), so inspect a stopped node. The
+report goes to stdout; log lines go to stderr.
+
+```
+./build/bin/slave inspect --datadir ./qkc-data/devnet
+```
+
+```
+shard 0x00000001 (qkc-data/devnet/shard-0x00000001):
+  genesis block:         0x661b12d25851f510519f8b157b2b76c95ea8ba4faf2a78f047c12c0bec792667
+  height:                0
+  state root:            0x76b7e413ee8a10d27ad5158ce91b8b8e61d6af8805b965a4ce11b93db0286ed1
+  coinbase:              0x000000000000000000000000000000000000000000000001
+  coinbase amount:       token 35760 = 3250000000000000000
+  evm_gas_limit:         12000000
+  evm_xshard_gas_limit:  6000000
+  hash_prev_root_block:  0x5ad443efb7cf5246a3d1bbc1734bd02bf3a5d83bedeccfcfe707d0ebee03780d
+  xshard cursor:         root=0 minor=0 deposit=0
+  chain id:              110001
+  fork schedule:         byzantium=0 constantinople=0 eip150=0 eip155=0 eip158=0 homestead=0 petersburg=0
+  head block:            none recorded (stub chain persists no head)
+shard 0x00040001 (qkc-data/devnet/shard-0x00040001):
+  ...
+2 shard(s) inspected, 0 failed
+```
+
+Nothing is printed for a shard until everything the report would assert has been
+checked, so a database that fails is described by its error alone rather than
+having its fields presented as that shard's genesis:
+
+- **The block must hold together.** A minor block's hash is its header's hash
+  alone, and without the cluster config there is no derived encoding to compare
+  against, so the meta is rehashed and checked against the `hash_meta` the header
+  commits to. Otherwise a database whose meta was replaced would report the
+  original, authentic-looking block hash next to a substituted state root. Block 0
+  must also be block 0, with an empty body.
+- **The block must belong here.** The stored block names its own shard through its
+  branch; a chaindb sitting in another shard's directory is reported as a misplaced
+  chaindb.
+- **A missing block must be the only thing missing.** The genesis block is written
+  last, once the chain stands, so its absence is an interrupted bootstrap —
+  `genesis block: none (bootstrap never completed; next boot re-initializes)`, and
+  the next `slave` run re-runs the fresh path. A head pointer with no genesis under
+  it is not a state this lifecycle produces, and is reported rather than described
+  as safely re-initializable.
+
+The EVM rule set is stored apart from the genesis block, keyed by its hash, and is the
+other half of what a reopen is checked against — so it is reported too: the shard's
+chain id (`BASE_ETH_CHAIN_ID + CHAIN_ID + 1`) and its fork schedule, which for every
+QuarkChain shard sits entirely at block 0. A datadir initialized before the rule set was
+written prints `rule set: none stored`; that one is recoverable, and the next `slave`
+run warns and writes it rather than refusing to boot.
+
+A report that cannot be written fails the command instead of being summarized as a
+success.
+
 ## Fixtures and pyquarkchain cross-validation
 
 Both real (singularity) cluster configs are checked in under
@@ -133,3 +196,35 @@ genesis block hash, byte-identical to pyquarkchain's
 its allocated state are derived by [`qkc.CreateMinorBlock`](../../qkc/genesis.go)
 and can be cross-validated the same way as the root genesis (see that README's
 [Pinned minor-genesis values](../../qkc/config/singularity/README.md#pinned-minor-genesis-values)).
+
+## Follow-up integration checklist
+
+The slave provides the process, per-shard database ownership, and the lifecycle
+around a stub chain. The following replacement points are deliberate:
+
+- **Real shard chain:** when the geth-core shard chain
+  ([#1](https://github.com/QuarkChain/goshard/issues/1)) exists, inject its
+  `ChainService` from `cmd/slave` instead of the stub. The seam already takes the
+  arguments `NewBlockChain` needs; `Stop` must drain every chain-owned goroutine
+  before the shard database closes, and the chain must refuse to open on a
+  missing head state.
+- **Genesis storage:** the same task retires the single `QKC-genesis-block` key —
+  it is scaffolding, the block under it is not. Block 0 moves into the chain's own
+  block storage (canonical-hash mapping and head pointers), and the key is dropped
+  rather than migrated: the databases hold genesis-level data only, so a clean
+  re-bootstrap is the whole migration. `slave inspect` then reads block 0 and the
+  real head through those accessors.
+- **Compatibility check placement:** geth checks rule-set compatibility against
+  the persisted head header *before* constructing the chain, and answers an
+  incompatibility by rewinding rather than refusing to start. Both become reachable
+  once the chain persists a head; move `ReconcileChainConfig` ahead of construction
+  then.
+- **Integration tests:** move the boot/reopen, inspect, mismatch, and goleak
+  coverage onto the real chain, and add a case where the rule set changes without
+  changing block 0. Keep the goleak allowlist empty for slave-owned goroutines and
+  fix their `Stop` path instead of ignoring them.
+- **Master-driven creation:** when the cluster protocol
+  ([#5](https://github.com/QuarkChain/goshard/issues/5)) lands, replace eager shard
+  creation with the pyquarkchain-compatible `PING(root_tip)` trigger while keeping
+  partial-boot rollback, idempotent blocking shutdown, and the `DBDirName` datadir
+  convention.