feat(upsert): add onConflict option for custom conflict columns

[jbingen]

Summary

Allows specifying custom columns for ON CONFLICT in upsert operations, instead of defaulting to the primary key. This is useful when upserting based on unique constraints other than the primary key.

Motivation

When you have a table with a unique constraint on columns other than the primary key, the current upsert always uses ON CONFLICT (primary_key). This PR adds an onConflict option to specify which columns should be used for conflict detection.

Example

// Upsert based on unique constraint (workspace_id, email) instead of primary key (id)
await tx.mutate.workspace_invitations.upsert(
  { id: '123', workspace_id: 'ws1', email: 'user@example.com', role: 'member' },
  { onConflict: ['workspace_id', 'email'] }
)

This generates SQL like:

INSERT INTO workspace_invitations (id, workspace_id, email, role)
VALUES ('123', 'ws1', 'user@example.com', 'member')
ON CONFLICT (workspace_id, email) DO UPDATE SET
  id = '123',
  workspace_id = 'ws1',
  email = 'user@example.com',
  role = 'member'

Changes

• zero-protocol/push.ts: Add optional conflictColumns field to UpsertOp schema
• zql/mutate/crud.ts: Add UpsertOptions type with onConflict field, update TableCRUD and TableMutator types
• zero-server/custom.ts: Use custom conflict columns in SQL generation when provided
• zero-client/crud.ts: Pass options through the CRUD executor

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