fix(BUG-009): require explicit nullable when alterColumn changes data type

[s2x]

Summary

Fixes silent data constraint corruption in alterColumn(). When calling alterColumn() with newDataType but without specifying nullable, the method silently made columns NOT NULL because null was mapped to 0.

Problem

// Column score is nullable=true
$coll->alterColumn('score', newDataType: ZVec::TYPE_FLOAT);
// BUG: score is now nullable=false - silent data constraint corruption!

Solution

Throw ZVecException when newDataType is set without explicit nullable:

$coll->alterColumn('score', newDataType: ZVec::TYPE_FLOAT);
// Throws: nullable must be explicitly specified when changing data type.

// Correct usage:
$coll->alterColumn('score', newDataType: ZVec::TYPE_FLOAT, nullable: true);

Changes

• src/ZVec.php:474-484 - Add guard clause in alterColumn()
• tests/bug_0009.phpt - Test exception is thrown, explicit nullable works

Tests

All 136 tests pass. Closes #56.