feat: placeholder entities for nullable has-one (alternative to #15)

[matej21]

Summary

Alternative approach to #15. Instead of returning null for disconnected nullable has-one relations, this keeps the placeholder entity pattern from the original contember-oss binding.

Why not return null (#15's approach)?

• Placeholder entities are the established binding pattern — they allow writing to disconnected relations (e.g. setting fields on a placeholder creates the entity on persist)
• Returning null forces ! assertions everywhere in components (<HasOne field={article.author!}>) and requires a separate $hasOne('fieldName') string-based API for mutations
• The type system already correctly maps nullable has-one to HasOneAccessor<NonNullable<T>> (never null)

What this PR does instead:

• $isConnected boolean — convenient getter on HasOneAccessor for checking connection state (alternative to $state === 'connected')
• Fix nullable: true in schema definitions — entity types say author: Author | null but schema definitions were missing { nullable: true }, causing $remove() to call $delete() instead of $disconnect()
• Placeholder behavior tests — 5 new tests documenting that nullable has-one always returns an accessor, never null

API pattern (unchanged, just better documented)

// Reading — always safe, placeholder returns null for field values
const authorName = article.author.name.value ?? 'N/A'

// Check connection — use $isConnected or $state
if (article.author.$isConnected) {
  // author is a real entity
}

// Mutations — directly on the accessor (no $hasOne needed)
article.author.$connect('author-2')
article.author.$disconnect()

// Writing to placeholder — creates entity on persist
article.author.name.setValue('New Author')

Test plan

• Typecheck passes
• 5 new placeholder behavior tests pass
• Full suite: 1388 pass (2 more than main), 58 fail (2 fewer than main — nullable: true fix helps)
• Pre-existing failures unchanged (browser tests + form has-many)

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