docs(field-trials): FT17 (schema-diff) report — ADR-0009 implementation

[hideyukiMORI]

ADR-0009 (#416) draft 直後の implementation trial 報告。Trial Issue #417。

結果

• `cli/schemaDiff.php` + `SchemaDiffer` (pure-static engine) + `SchemaCompiler` 4 helper を private → public
• MySQL (`INFORMATION_SCHEMA`) / SQLite (`PRAGMA table_info`) 双方サポート
• 5 live scenarios 全 pass: in-sync / new-column / round-trip / extra-column warning / drop-table → CREATE TABLE 復元
• 10 unit cases (128 → 138)
• 全 lint (test / analyze / format:check) green

F-N (5 件)

• F-1 (info): SchemaCompiler 4 helper を private → public 化
• F-2 (info): MySQL upper / SQLite lower case を introspection 関数内で handle
• F-3 (ergonomics): `composer schema:diff` alias 追加要
• F-4 (ergonomics): `--help` expand を docs PR で
• F-5 (medium feature-gap): Add-index path 未実装 → 別 follow-up issue

Aftermath plan

1. Feat PR: SchemaDiffer + cli/schemaDiff.php + SchemaCompiler promote + composer alias + tests
2. Docs PR: docs/development/schema-migrations.md + AGENTS + ADR cross-link + --help expand
3. Follow-up issue: add-index path (小、follow-up trial 不要)

Closes #417.

Test plan

• 5 live scenarios on MySQL
• composer test 138/138 (128 → 138)
• composer test:http 24/24 (1 expected skip)
• composer analyze (Phan) exit 0
• composer format:check exit 0