🔍 What was changed

Adds first-class upsert support as a state on InsertQuery, not a new query class.

• InsertQuery::onConflict(OnConflict|string|array) — new setter. Presence of state flips getType() from INSERT_QUERY to UPSERT_QUERY.
• Cycle\Database\Query\OnConflict — immutable value object describing conflict-resolution policy: target columns, doUpdate(null|list|map) (supports column expressions via FragmentInterface), doNothing().
• Driver-specific subclasses (mirroring the CursorOptions pattern):
  • Postgres\PostgresOnConflict::onConstraint($name) — ON CONFLICT ON CONSTRAINT.
  • MySQL\MySQLOnConflict::withRowAlias($alias) — customize the INSERT ... AS <alias> row alias.
  • SQLServer\SQLServerOnConflict — placeholder for future MERGE-specific options.
  • Each provides static from(OnConflict) that accepts base or self and rejects mismatched subclasses.
• Compiler dispatch via new CompilerInterface::UPSERT_QUERY constant:
  • Base Compiler::upsertQuery() → SQLite (INSERT ... ON CONFLICT ... DO UPDATE|NOTHING).
  • PostgresCompiler →z same + RETURNING + constraint target.
  • MySQLCompiler → INSERT ... AS <alias> ON DUPLICATE KEY UPDATE (requires MySQL 8.0.19+).
  • SQLServerCompiler → MERGE ... WITH (HOLDLOCK) WHEN MATCHED THEN UPDATE WHEN NOT MATCHED THEN INSERT [OUTPUT].
• CompilerCache::hashUpsertQuery() — caches single-row upserts, parallel to single-row insert caching. Hash delegates to polymorphic OnConflict::getCacheKey() so driver-specific fields are correctly disambiguated.

// Portable
$db->insert('users')->values($row)
    ->onConflict('email')                            // shorthand: DO UPDATE on all columns
    ->run();

$db->insert('users')->values($row)
    ->onConflict(OnConflict::target('email')->doUpdate(['name']))
    ->run();

$db->insert('counters')->values(['key' => 'x', 'n' => 1])
    ->onConflict(OnConflict::target('key')->doUpdate([
        'n' => new Expression('counters.n + EXCLUDED.n'),
    ]))->run();

$db->insert('logs')->values($row)
    ->onConflict(OnConflict::target('request_id')->doNothing())
    ->run();

// Driver-specific
$db->insert('users')->values($row)
    ->onConflict(PostgresOnConflict::onConstraint('users_email_unique')->doUpdate(['name']))
    ->run();

🤔 Why?

• Upsert is, in SQL terms, an extension clause on INSERT for 3 of 4 drivers — modelling it as a separate query class duplicated into/columns/values machinery and required a parallel Database::upsert(), Table::upsertOne(), BuilderInterface::upsertQuery() surface.
• Driver-specific behaviour (Postgres ON CONSTRAINT, MySQL row alias, SQLServer MERGE quirks) doesn't belong on a single base DTO — handling it with runtime CompilerExceptions pollutes the abstraction. Driver-specific subclasses move the check to type level and match the existing CursorOptions pattern in the codebase.
• MySQLCompiler uses modern INSERT ... AS new_row ON DUPLICATE KEY UPDATE col = new_row.col rather than the deprecated VALUES(col) form.

📝 Checklist

• Closes 💡 Add support for MySQL: ON DUPLICATE KEY UPDATE, PostgreSQL: ON CONFLICT #50 (and supersedes the architecture proposed in feat: Added support for upserting data #231)
• How was this tested:
  • Unit tests added (OnConflictTest, PostgresOnConflictTest, MySQLOnConflictTest — 27 new tests covering DTO semantics, immutability, from() narrowing/rejection, cache-key stability)
  • Functional compile-time tests added per driver (Common, SQLite, MySQL, Postgres, SQLServer — exercising shorthand, explicit doUpdate columns/expressions, doNothing, multi-row upsert, constraint target, custom row alias, cross-driver subclass rejection)
  • Functional runtime tests against live MySQL/Postgres/SQLServer servers — left for CI; SQLite runtime path is covered

📃 Documentation

• Database::cursor()-style PHPDoc on InsertQuery::onConflict() and OnConflict describes both shorthand and DTO forms.
• InsertQuery::run() PHPDoc notes that lastInsertID is unreliable on the update branch for MySQL/SQLite without RETURNING — driver-specific subclasses with ReturningInterface (Postgres, SQLServer) are recommended for code that needs the row identity back.

Known scope limitations (follow-ups)

• where() clause on DO UPDATE (Postgres) / WHEN MATCHED AND ... (SQLServer) is intentionally deferred — would require integrating WhereTrait into the immutable DTO; the subclass hierarchy is ready for it without polluting the base.
• MySQL < 8.0.19 fallback to legacy VALUES(col) not implemented (current target: MySQL 8.0.19+, the row-alias syntax cutoff).