feat(cli): add export/compact/check subcommands (#18)

[robertoberto]

Summary

Adds three new datawal-cli subcommands that round out the 0.1.x CLI surface without touching the source store on disk.

Subcommand	Synopsis	Source on disk
export STORE OUTFILE	Live KV projection → JSONL via DataWal::export_jsonl. Refuses to clobber an existing OUTFILE (exit 1).	not modified
compact STORE TARGET	Snapshot-style rebuild into TARGET via DataWal::compact_to. Refuses non-empty target (exit 1).	not modified
check STORE	DataWal::open + get every live key (per-record CRC revalidation through the fd pool) + RecoveryReport. Exit 3 on hard get failure; exit 2 on tail truncation / mid-stream error.	not modified

cli.rs top-doc and commands.rs top-doc are reframed accordingly: subcommands are now in two groups — read-only inspection (scan, get, report, verify, dump, check) and source-untouched mutations (export, compact). No put / delete / rotate in 0.1.x.

What stays the same

• Single binary datawal; positional store path; --json global flag.
• JSON schema string datawal.cli.v1 is unchanged. Three new kinds (export, compact, check) are additive — no field renames, no encoding changes.
• Cooperative single-writer lock: every subcommand still goes through DataWal::open / RecordLog::open.
• Exit-code grammar (0/1/2/3) extended additively: 1 now also covers export/compact clobber refusals; 3 now also covers per-record get failures during check.
• Reserved CLI names (query, select, where, index, server) remain unused.

Tests

+6 integration tests in crates/datawal-cli/tests/integration.rs (33 total, 0 failures):

• export_writes_one_jsonl_line_per_live_key
• export_refuses_to_overwrite_existing_outfile
• compact_writes_to_empty_target_and_emits_stats (also reopens compacted target and confirms both live keys resolve)
• compact_refuses_non_empty_target
• check_clean_store_succeeds
• check_detects_tail_truncation (appends 8 junk bytes to the active segment, asserts exit 2 + non-zero tail_truncated)

Local validation

All six gates green on feat/cli-export:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets (full workspace; CLI suite: 33 passed)
• RUSTDOCFLAGS='-D warnings' cargo doc --workspace --no-deps
• cargo bench --workspace --no-run
• cargo publish --dry-run -p datawal --allow-dirty

Closes / Refs

Closes #18 (CLI-2 remainder; CLI-1 was closed by PR #24).
Refs #7.