Replay upstream state_machine_tests.zig scenarios against the OCaml core

[devin-ai-integration]

Summary

Adds the first automated OCaml-vs-TigerBeetle comparison (follow-up to closing #7). ocam/test/zig_corpus.ml parses every check(...) table in the retained ocam/src/state_machine_tests.zig (byte-identical to the pinned upstream copy) and runs it against State_machine. It mirrors check_version in the Zig file:

• the table grammar from testing/table.zig (_ = default, leading letter ignored, -n = maxInt - n);
• expected-reply construction (timestamps, exists echoing the original or rolled-back timestamp, pending-id fill-ins);
• the prepare/pulse clock, with batch_max.create_transfers = 30 from the test_min config.

Status codes are read from src/tigerbeetle.zig at runtime and matched against Result_code.*_codes, so coarse OCaml statuses still count as matches.

The report is diffed against test/zig_corpus.expected on dune runtest. Parity progress and regressions therefore show up as a diff, and dune promote accepts an intended change. Current baseline:

41 scenarios: 18 pass, 18 fail, 5 skip
create results: 1 status mismatches, 79 timestamp-only mismatches

The failures point at two real divergences:

1. Error results return timestamp = 0. Upstream returns the event's commit timestamp for every non-exists result, including linked_event_failed. In OCaml this comes from error status = { timestamp = 0L; ... } in state_machine.ml. This causes almost all of the failures.
2. imported events: imported batch. OCaml rejects an imported transfer with imported_event_timestamp_must_not_regress because it checks against the global commit_timestamp. Zig checks against the transfers groove's key_max, i.e. per object type.

A SKIP means the scenario uses something the OCaml types can't express yet: CDC (get_change_events), reserved/padding bits, or statuses with no OCaml equivalent (exists_with_different_user_data_128, imported_event_timestamp_must_postdate_debit_account).

Supporting changes:

• State_machine.unsafe_replace_account_for_testing implements the harness's setup action, which overwrites balances directly.
• The tests stanza now lists modules explicitly so the corpus runner stays a separate executable.
• README and OCAML_REWRITE are updated.

Checked locally on OCaml 5.2.1 with ocamlformat 0.26.2: dune build, dune build @fmt, dune runtest and opam lint all pass.

Link to Devin session: https://app.devin.ai/sessions/1ffda120da2640bc8996571fcff0a87c
Open in Devin Desktop: https://app.devin.ai/desktop/session/1ffda120da2640bc8996571fcff0a87c?variant=devin
Requested by: @gpu004

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring