Prove native transactional table capabilities

[chubes4]

Summary

Follow-up to #393 for #377. Provide the transaction guarantees needed by identity reservation before advertising a backend capability.

The maintainer approved the capability as a backend-neutral integration API. This is not a claim of unchanged-consumer MySQL implementation parity. The temporary scope separation is superseded; the API is restored at 34057b03e535a0324802030ebed1118b53023d2f, whose tree is identical to the tested dcfe6b4 tree.

Consumer adapter: Extra-Chill/data-machine#3485. Land this engine prerequisite first.

• Admit native reads and writes under the canonical root lock; retain admission across explicit transactions, rollback and savepoints.
• Prevent independent SQL readers from observing uncommitted JSON/Markdown changes, and refresh stale snapshots at subsequent admission even when the reader never waited.
• Expose optional wpdb::supports_transactional_tables(array $tables): bool for exact table sets. Require recognized canonical providers, matching mutation runtimes, admitted roots and permanent registered tables. Reject unknown, shadowed, arbitrary-provider, outside-root and read-only configurations.
• Reuse typed scalar evaluation for duplicate-key increments required by identity reservation.
• Add independent-process transaction, crash recovery, reader visibility and advisory-lock conformance tests.

This is bounded coarse serialization, not MVCC, an InnoDB engine label or physical mysqli support. Existing five-second lock waits can return explicit contention failures. Direct filesystem writers bypassing native locking are outside this contract.

Verification

Tested MDI: dcfe6b467d9d509271204faa313fc0cefa35c444.
Companion Data Machine candidate: 4432c2433dbce2985654cc88f57dc3d5abc411f0.

Repo-native reproduction (PHP with process-control support for independent-process probes):

php tests/probe-native-identity-reservation-transaction.php
php tests/smoke-native-transaction-read-isolation.php
php tests/smoke-native-advisory-lock-processes.php
php tests/smoke-native-journal-ownership.php
php tests/smoke-native-wpdb-lifecycle.php
php tests/smoke-native-request-snapshot.php
php tests/smoke-native-table-upsert.php

Paired unmodified DME consumer suite, WordPress 7.1 / PHP 8.3, managed multisite PHPUnit with the shared WP-CLI bootstrap:

Backend	Passed	Failed/Error	Skipped	Total
Native, with companion consumer adapter	1040	4	9	1053
MySQL control, with the same consumer adapter	1046	0	7	1053

All 15 prior identity/upsert failure identities disappear. The four remaining failures require physical mysqli connections/handles. Dedicated Data Machine reservation tests also pass: 26 passed, 0 failed, 1 skipped / 27 on MySQL.

Operator-retained runs: transaction-capability-full-native-dcfe6b4-4432c243, transaction-capability-full-mysql-dcfe6b4-4432c243, and dm-post-identity-mysql-4432c243-dedicated. Each retains its recipe and full JUnit. These are operator artifacts, not publicly hosted links; the commands above reproduce this repository's conformance checks.

The pre-existing smoke-native-table-replace.php has two failures on both the merged baseline and isolation candidate; this PR does not claim to fix them. Full MySQL parity remains tracked in #377. No release or deployment is included.

AI Assistance

OpenAI GPT-6 Astra (openai/gpt-6-astra) via OpenCode investigated the requirements, implemented and reviewed the engine and capability changes through isolated agents, corrected cache and capability-boundary defects during review, and ran independent-process and paired native/MySQL validation under maintainer direction.