feat(fsck): add --page-size override, --help, and split exit codes

Split pagedb-fsck.rs into a directory module (cli/run/main) so grammar
parsing, execution, and exit-code policy are separately testable.
Adds --page-size for stores created at a non-default page size and a
--help/-h flag, and gives usage errors, an unreadable store, and an
actual integrity failure distinct exit codes (2, 3, 1) instead of one
generic non-zero.

Author: farhan-syah

Cargo.toml

@@ -142,7 +142,7 @@ fluxbench = "0.1"
 
 [[bin]]
 name = "pagedb-fsck"
-path = "src/bin/pagedb-fsck.rs"
+path = "src/bin/pagedb-fsck/main.rs"
 
 # These are executable benchmarks, not test harnesses. The cross-engine suite
 # is a separate non-default workspace package so normal PageDB tests never

README.md

@@ -67,9 +67,19 @@ cargo run --bin pagedb-fsck -- <path> --deep \
 
 The KEK may instead come from `PAGEDB_KEK` and defaults to all zeros. The realm
 defaults to all ones; nodedb-lite stores use the all-zero realm shown above.
-Argument parsing rejects duplicate flags, unknown options, missing realm
-values, and multiple positional KEKs. Inspection disables commit-history
-retention and does not rewrite authoritative `main.db` or segment bytes.
+Add `--page-size` for a store created at anything other than 4096 bytes, and
+`--help` for the full grammar. Inspection disables commit-history retention and
+does not rewrite authoritative `main.db` or segment bytes.
+
+Arguments are validated before the store is touched, and each failure class
+gets its own exit code so a caller can tell them apart:
+
+| Code | Meaning |
+| ---: | --- |
+| 0 | Opened cleanly; with `--deep`, the report was clean |
+| 1 | An integrity problem was found |
+| 2 | The command line was invalid; the store was never opened |
+| 3 | The store could not be opened, or the report could not be written |
 
 ## Benchmarks