feat: nullable has-one returns null, add $hasOne for mutations

[vparys]

Summary

• Nullable has-one relations (author: Author | null) now return null from entity field access when disconnected, instead of always returning a HasOneAccessor with a placeholder entity
• New entity.$hasOne('fieldName') method always returns HasOneAccessor regardless of connection state — for mutation access ($connect, $disconnect, $reset, etc.)
• HasOneHandle.hasOne() delegation for nested $hasOne access (e.g. entity.author.$hasOne('avatar'))

API pattern

// Reading — null-safe with optional chaining
const authorName = article.author?.name.value ?? 'N/A'

// Mutations — always available via $hasOne
article.$hasOne('author').$connect('author-2')
article.$hasOne('author').$disconnect()

// Dirty state — via handle
const isDirty = article.$hasOne('author').$isDirty

Test plan

• Typecheck passes (bun run typecheck)
• All 21 has-one relation tests pass with updated patterns
• Full test suite: 1376 pass, 0 new failures (9 pre-existing: 8 browser, 1 form has-many)

[vparys]

Fixed in #16.